freckles if they're doing it by volume (yuck! at least everything in the UK is done by weight in g, not cups, or oz!) they can know that when you get it out of the packet it has a certain volume. If you microwave it, or boil it, or fry it or whatever, it's likely to reduce in volume by a different amount for each cooking method, and potentially the amount of time used to cook it, therefore they're giving you the measurement in the known state before you've cooked it.

Makes sense if you think about it!