I know exactly what you mean.
I'll point out some things (including some stories I feel are related in a way, although not completely aligned with your question I just retroactively realized) then answer your questions to the best of my ability with my hypothesis.
I feel this happens many times in online conversions but I actually feel like it happens a lot in real life as well, or at least with some of the people I have to deal with. A lot of times it depends upon who you're dealing with as I'm sure you would agree and what they are doing at that same time (if online, perhaps talking to multiple people).
Here's an example of what happened to me with a certain person in real life that really, really bothered me. I am curious if anyone can relate. Let me set up the context first:
Towards the end of a previous job I had, my boss, who hired me to
consult with him for internet marketing. I did quite a good job at
this and even let him try most of his ideas first and tried to be
positive about them, he even told me verbatim "you saved us from
getting kicked out of X huge nationwide retailer because of your idea
and campaign." I'm trying to be discrete here...but just think of the
"biggest" and most notorious in certain ways retailer...with very low
prices. Usually these prices are everyday :). I also had let him try
out many of his ideas with my attempted positive aid, which did fail
for about 4 months until we fully did mine.
In essence, I had a lot of credibility with this guy...a lot.
So towards the end of my relationship with him and that job, he started acting...to put it in the best way I can...unusual.
Among other things and closer to your point though here is how our conversations became:
Boss talks and gives me a very long dialogue. It includes:
Here's the problem, many times on these "thing/idea/problems" that he would speak of he would be missing very important concepts or just downright state it whatever he was saying completely wrong like he didn't know what was going on (this obviously worried me). For instance let's say he did that on #2, #3, #5, #7, & #10.
The things he would miss or not understand were vital to making the business work. Extremely vital. Now, normally when someone is talking, you don't want to interrupt them because it's rude.
However, I find myself not able to remember all of the things he just mentioned (all 11 of them) and then at the very end tell him my responses to each of the ones that needed correction (and each response I had was not very short, but was polite; although if he was completely off it would take me a second for me to 1) Explain to him everything & 2) Convince him what I was saying was true.
So I would have two options here:
A) Let him keep talking and try to respond to each of those points (each of which was extremely important) at the very end of when he was done saying all of them. This was hard for me as I could not remember all 11 of them and then remember the 5 that were wrong, what he said exactly, and also what my brain had just told me when he was mentioning that topic what I needed to tell him. If you think about it, it's a bit of a memory overload. So, I simply could not wait until the end to tell him things that were vital to our success that I did not want to forget about.
B) I could attempt to interrupt him on each point even though the way he spoke was in a way where he'd say a lot about one of the topics then never even pause as he transitioned to the next one. I could never find a time or opportunity to respond. Since A (above) wouldn't work, and he wasn't giving me the opportunity to respond to what he was saying...I unfortunately had to weigh interrupting him (with apologies and in a polite as possible manner) against him not getting this information he needed to know very bad and could dramatically affect the success of the business.
Keeping the business in mind and the fact that a few months earlier he actually got angry at me for NOT telling him something I knew he needed to know/thought would fail, I felt I had to, unfortunately, interrupt him.
Here's the thing, towards the end of my relationship with him he started saying "STOP TALKING WHEN I'M TALKING" while listing 11 points like I explained above if I tried to interrupt him - which as I explained I was essentially forced to do. After saying this wouldn't wait a millisecond and would continue with the barrage of information...much of which, as I explained was incorrect or had huge holes in it.
He would be very rude about it, and keep repeating that exact phrase until I could not share the important information with him, and would do it repeatedly...over and over again even if he was 180 degrees on one of his points (many times his points were quite elaborate too). I couldn't do option A as I couldn't remember all the content and I even almost felt like he didn't want to hear what the reality was about internet marketing anyways from someone he gave so much credibility to beforehand....which was strange to me.
Anyways this new thing he did of "STOP TALKING WHEN I'M TALKING" really, really bothered me a lot. A LOT. It was very rude and unprofessional. It was probably one of the rudest things I've ever encountered someone do to me in my opinion or at least strange as....why would you be paying me to not share my opinion with you that you had previously (and many times actually) admitted beforehand was extremely credible?
Anyways, that relationship ended as his behaviors became more and more unusual...sometimes even schizophrenic in a way.
But, that's not the only time that that happens to me. Admittedly I think I have some sort of verbal timing issue with interrupting people during verbal communication but I always try to be extremely polite about it and if they were in the middle of something let them continue. It's like I really want to add to their point or something and know I have very valuable information, but they just keep going on and on with no pauses or opportunities for me to do so. This happens a lot with certain members of my family as well too - it's as if they don't want to hear my response of what they just said about me as a person because (this is one of the theories I have, that they are afraid I may prove them wrong)..which isn't necessarily always the case. It may be that they say something that is essentially "misaligned" and then base their actions upon that, but do not give me a chance to respond and actually "realign" what they had said.
Not allowing someone to respond to something someone said to me is either
A) I am holier than thou syndrome
B) They are scared/worried of the potential I am right, but most of the time, unless it really affects me or my finances personally OR I'm on Stack Exchange (I made an exception on this website to try to encourage people to debate things so we can get to the actual truth...and because I don't want stray people coming in from google searches to be misinformed....thus I'll challenge them quite intensely at times...this usually results in downvotes btw).
C) They'll delete something I said in my rebuttal and/or not come up with a rebuttal of their own...in fact NO-ONE has ever tried to rebuttal me, which bothers me because why can't we debate for the sake of knowledge? Debates motivate people to be "right"...and usually end up getting towards the real "truth." That's where the value is for everyone. However, flaming has to be controlled.
Most people are cool though in real life and I really have no problems with them as they'll give me an opportunity to respond.
If you notice in many of my posts I try to consider everything that is said so I can provide a proper response to it - as I hate it when I'm barraged with some form of communication that I cannot properly respond to. That's one of the reasons I love getting to sit here and type out my responses. Time isn't a factor. I can respond to each piece.
I'll stop belaboring (which could actually be an answer to your question ironically) and get straight to your questions:
#Questions and Answers
Main Question: Why do people tend to reply to only one topic when
multiple topics are brought up simultaneously?
Verbally in person: In the case of the story I just told about my previous boss, I actually cared A LOT about the success of the business and was very invested in it time, energy, and money wise. I was a lot getting paid very little for what I had done, was doing, and the rarity of some of my specific skillsets that have a lot of demand with online marketing (not to sound conceded on that last one).
Because I cared so much, I tried to respond to everything I felt was important. But I don't think many or most people have that same level of passion or drive...they would probably have tried to go with option "A)" in that case which would of been to wait til the entire dialogue is finished then, due to the limit of short term human memory, just respond to one thing.
In fact, I see A LOT of people do that all the TIME in real life - if I feel I have valuable information or something really and truly related to a topic they are going on about...I'll try to interrupt them, politely of course..but it's kind of painful. I guess some people either don't have the courage, tenacity, or motivation to do so even when it's directed at them as a person (in which case flaming, name calling, and labeling usually come into play which is kind of sad...but hey they couldn't remember everything that was just communicated to them so they'll just pick the last topic then be angry for that remaining time) or would directly impact their lives....either that or I just have bad social skills and interrupt people at inappropriate times.
Now, online I think it's slightly different at times.
I'm on the computer all day and even though at mostly the chatter, many times in order to actually focus on something and get it done instead of trying to respond to everyone which used to happen as an affiliate marketer from affiliate managers I'll just close all of my instant messaging programs. Otherwise I feel compelled to respond as best as I can because I really hate it when people DON'T do that. I think that's probably a healthy tactic more people should use.
Otherwise, they will be online and trying to get work done in a semi-procrastinatory way by having a chat open for micro breaks and sometimes both people are O.K. with that and understand that understanding between each other.
The other answer is, sadly enough, VALUE. How much does that person see you as being valuable to them in their lives...what do you really have to offer that can make them more money, have better/get more relationships with the opposite sex, resolve x or y health problem, etc.?
Yes, it's very irritating as I've gone from being completely ignored on AIM to being a semi-celebrity within their entire office to being completely ignored again in the internet marketing field just based upon how much money I'm bringing that person and or company in. Sadly, a lot of people really are in it for themselves (especially in my field). It is human nature afterall. I have noticed though, that the people that succeed are the ones that see true - real value in people and not such in at that moment in time. I've had occurrences where I won't be making him any money, but my buddy will still respond just the same to me as if I was. That's an important trait I recognize. It means that he understands the value of relationships - and much more so than just from a selfish perspective.
Imagine if you were with President Obama on the phone and at the same time your girlfriend and Mom were trying to chat with you on AIM...you may not be able to answer everything and you have to prioritize something as you can't do everything at once, even if you're trying to do it in a rotational fashion.
This is a situation more frequent in online conversations, where you
can type many things before the other person has a chance to reply.
Perhaps cognitive load, multi-tasking ability, have something to do
with it? At any rate, it does seem like the person "ignores" the other
unanswered question at times.
As I said I do think a lot of it has to do with multi-tasking ability - but a lot of it also has to do with your value to that person at that point in time.
Once again, let's say you're about to go on your first date with someone that you really like....but your ex whom you also really like wants to hang out instead as they changed their "feelings" (or actually many times this could mean that they feel like their value is low at that point in time and thus, because they know you (you as in just talking 2nd person not meaning the OP) may and probably will or no one else is responding to them so...they rationalize it in their minds and then to you with a manipulative reply.
It partially has to do with your value to people at that point in time...and those values are really never static in my opinion although they may sometimes tend to lie within certain ranges.
I believe I've been pretty thorough, ironically, lol.
I hope that helped to answer your question, if not please let me know by responding in a comment with @Taal :) If you'd like some research I can try to find some but this sort of specific type of question doesn't lend well to empirical research I can find - sometimes you never know though. Otherwise don't forget to accept it as your final answer if you choose it to be the one as I need the points to hit 1000 :)