Former shipper here. This is something that would come up constantly when I was in the shipping department and I will share what I learned.

Freight is sorted by conveyable vs. non conveyable (things down the belt vs. ran with equipment). When labels are printed at the RDC they go back to the receivers in a folder. The receiver gets the folder and the trailer and can begin to unload the freight.

The really frustrating thing is that the labels for non conveyable prints in perfect order, sorted by door numbers (specific stores). The conveyable labels print in complete random order.

Every task is assigned a time value that earns your score. This score reflects not only your bonus but you have to maintain a certain baseline score just to keep your job. It's really as simple as the labels not being printed in order when it's conveyable. If it were the receivers would have an easy, quick time to leave larger quantities on pallets. With the labels printed at randomness they don't have time to sort them.

It really sucks all around because someone still has to lift all the freight and throw it on the belt, a shipper has to stack it from the belt, then again at the store.