You know how it is? You get to the till and you carefully put all your shopping out on the moving strip so all the heavy stuff is at the front. That way packing it at the other end is easy and you don't end up dropping your washing liquid on top of your cakes...
...and then the checkout operator proceeds to pick up the items seemingly at random, as if they've seen your game and are determined to stop it right now before you get any clever ideas.
So: Is this deliberate? Is this one of those ways to make a dull job go by more quickly?