Grab a piece of un-ground beef. There's red stuff and white stuff in it. The red stuff is muscle tissue, called "meat" or "lean". The white stuff is adipose tissue, called "fat". When the butcher makes ground beef, he estimates the amount of muscle tissue and the amount of adipose tissue going into the grinder. 70/30, for example, is approximately 70% muscle tissue and 30% adipose tissue.
The red stuff is muscle tissue, called "meat" or "lean".
You answered the big question I had. Lean is an adjective, not a noun, but they use it as a noun.
Now, how much filler is in hamburger meat ?
Since the 'butcher' is 'estimating' the amount of fat vs. muscle tissue in the meat going into a grinder, then these 70/30 or 80/20 or 85/15 labels are not exactly accurate, are they ?