Your supermarket might pack their products tighter after grinding to use shelf space more efficiently. It could have more water and/or less fat than your butcher's ground beef as well.
The whole idea that we can now buy ground beef by its percentage of fat per weight instead by the pre-ground cut is a bit disconcerting. Even with the holdover, ground sirlion, there's the 90%, 93%, and 95% lean ground sirloins. I can't even pretend my 80% on the button is ground chuck.
"I knew you before you knew you had hands!" ~Tracey Brogan
Serenity Prayer - Calvin's Edition: For the strength to change what I can, the inability to accept what I can't, and the incapacity to know the difference. ~Bill Watterson