In my experience I tend to find low-fat products labelled as "99% fat free" much more often than the equivalent "1% fat".*
Why is this so? To me it seems counter-intuitive, because it reminds me of psychological pricing, where retailers price things slightly less than round numbers, partly so that consumers under-perceive the price. If that same effect is happening with the "99% fat free" labeling, consumers would over-perceive the amount of fat.
Is it because the psychological pricing phenomenon doesn't apply in this case for whatever reason? Perhaps other phenomena are at play? Or perhaps it's something that doesn't have to do with the numbers?
*: I can't find a source for this unfortunately, and I'm aware that in some circumstances the "1% fat" form is the norm, for example with low-fat milk.