What’s The Real Reason They Check Your Receipt At Costco?

I recently stopped at Costco & picked up a gallon of  milk. Yeah, I know..."You went to Costco & bought ONE item??" Milk's 2 bucks a gallon and it's on my way home from work. I stuffed the receipt in my pocket and headed for the exit. Oops! I have to show my receipt at the door. I dug it out & joked to the receipt checker, "I've got a big one for you!" You probably think they're trying to prevent theft. I thought so, too. After all, you can buy furniture, appliances, and $250.00 bottles of scotch there. The real reason they check your receipt, according to thetakeout.com, is to check for cashier errors. I'm pretty sure they weren't worried about me trying to shoplift a gallon of milk.