I think you're being too extreme. It quite possible to have a less than complete grasp of something and write down the wrong answer. Heck, you can give the wrong answer even when you know something cold, all thought that's obviously not the case here. We don't know how much or how little attention the kid is putting into it and this is a 5th grader, after all.

I understand what you're getting at but some kids don't get all of this all at once in a blinding flash - they become more and more familiar with it over time.