When I was working in the butcher shop one of my jobs was cutting hamburger meat. You cut the lean and weight it, then you cut up fat that has been trimmed and weigh it. For a 100# of 80/20 it's 80# of lean and 20# of fat.
>>Have you ever taken hamburger you purchased and verified these figures for yourself ?<<
That's a silly question. You can't do that but if you want to try go for it.
>>The real difference (I am told) is in how much it shrinks during cooking, and how much grease(fat) it produces(separates) via cooking.<<
Sounds like the people who "told" you that were rather naive. It depends on where the fat came from and how you cook it. Some fat will render out completely when the hamburger is cooked to very well done and dry other fat will not. Not all the liquid in the pan came from fat. Some of it is body fluids from the meat. Also, if you buy hamburger already ground and packaged the law allows them to add a certain percentage of water to keep the hamburger from drying out prior to cooking.
I never buy pre ground hamburger. I buy cuts of meat and grind it myself. I prefer some pork fat included.
For a 100# of 80/20 it's 80# of lean and 20# of fat.
Going by POUNDS of lean and POUNDS of fat makes sense. In the end, though, it's not 20% fat, it is more. Correct?
It would seem that you could weigh the hamburger before you cook it, and then after, and the difference would be the loss in fat and water.