Why prefer "99% fat free" to "1% fat"? 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.
If that same effect is happening with the "99% fat free" labeling, consumers would over-perceive the amount of fat


I think you are misunderstanding the desired effect here. I don't see how "99% fat free" would lead to the impression that a product contains a lot of fat. My read is, "This is 99% fat free! That's really good!" as opposed to "1% fat" which more emphasizes the fact that fat is still present than the fact that it's not. Said another way, because 99 is a far higher number than 1, the brain is more positive about getting something that is really good (much higher fat-free percentage) than something that is slightly bad (very small fat percentage)

The other thing you may be overlooking is that "fat free" is a marketing term in and of itself. Products claim to be "fat free" in order to attract dieters / those looking to eat healthy. When comparing products people may choose the ones which have actively tried to reduce the amount of fat; this is what the "fat free" term is intended to convey. Compare that with milk, where "1% fat" really means "1% milkfat", a technical description of how rich the milk is (where "whole milk" is 3.25% - 4% milkfat typically), so the terminology is not used as a means to advertise that product as being better than the competition but rather as a means to identify the content of the product itself.