I don't know if this is important, but at the start the pattern of the groceries are different.
Anyways, to everyone who doesn't get it after reading everyones else:
The guy keeps hearing the cellphone call and keeps buying the milk and all that. And the cashier is saying they aren't running out of milk anytime soon. When he gets home to find his decapitated mother, he calls the police repeatedly so that one guy is talking about how the other one keeps calling 911.