I retired from Costco...doesn't make me an expert, but it did give me some otherwise little know things.....
Meat prices fluctuate radically at certain times of the year, kinda like gasoline, just what day your there.
Best time to buy your hamburger is coming....around the Thanks-Xmas holidays, all the Prime Grade meat that gets to expiration point get ground up into Hamburger, still 80/20
just more Prime available during this time of the year do to demand.