I assume that the cheating student didn't know that he was copying answers from someone who was doing poorly. It was third graders after all; one wouldn't necessarily expect them to be able to pick the best target every time.
How the heck is that even possible? :o
Because what the cheater is trying to accomplish is to avoid having to think.
It's an act motivated by either laziness, apathy or rebellion (or some combination thereof). Not motivated by trying to get a good grade.