I was a cashier at Target for three years, and let me tell you....
We were trained in how to sort and bag items by type: put cold items together in one bag, produce in a bag, chemicals in a separate bag, double bag heavy items like gallons of milk, use the guests' bags if requested, etc. Most of my co-workers tried to follow the guidelines. But I also noticed that some of them continued to sort and bag things by "type" even when it wasn't necessary or desirable, like putting all the canned items in a bag (ouch!). You just get into a particular rhythm. But there were also some people who just didn't think through what they were doing. Or didn't really want to be bothered with doing things the way they were told. You know the type.
We sorted items in order to keep like with like. Most people don't want household chemicals near their food. Others don't care. Almost everyone wants to keep cold items cold as long as possible, or keep soft perishables (like tomatoes or bread) at the top of the bag so they won't bruise. On the whole, I think most of us tried to follow our training. But I must also say that while training materials may have been consistent, the importance of certain details was not consistently emphasized. In other words, some baggers (and some managers, too) really don't understand what all the fuss is about.
Also, remember that we must work very quickly to accommodate you and all of the other guests in line. Our transactions were timed and a fraction of a second in delay would adversely affect our performance reviews. The workstation is designed to allow us to ring up your items and bag them as quickly as possible. It was a very rare occasion when my guests could bag their items faster than I could. Remember that I have to placate the disgruntled guests that are still in line after you leave and who think I'm taking too long. Believe me, someone will complain. So it really helped when a guest gave me all of their bags before I started ringing things up so that I could arrange them and hopefully bag everything in a way that would make us both happy.
By the way, most of us on the floor either had college degrees, or were working on them. Never judge anyone's intelligence by their job. All legitimate work is honorable work.