@jarman ,
@sfslackey is right about time being the greatest evil. Back stock is a huge time eater, so to keep from getting in trouble for not finishing a task that really couldn't be finished properly in the first place people flex since that's quicker. And people get pulled away from their assigned task all the time, guests and backup. Add on reshop, since that makes someone bounce back and forth through their entire area rather than having things relatively together, and that is why fill a shelf and blackstock the rest is really damned hard.
The ease you describe is easy if all you have to do is fill shelves and nothing else. But there is no such thing, everyone has to fill all roles at once and can (and guaranteed will) be repeatedly pulled away from those shelves.