Coriolis,

I've often wondered this too, and the only thing I can think of is that it's easier to figure the weight of raw meat since it's often labeled on the package. After you cook it, the meat might look smaller or weigh differently, but the nutritional content in the meat itself will stay the same (correct me if i'm wrong here, anyone, this is just what i've always assumed). of course, don't forget to add oils and condiments used in cooking to the nutritional value.