The prefect problem may not be as much a contradiction as it is a problem but it fits this section better than anywhere else. The contradiction, or problem, however you term it, is partially caused by the first-person limited narration, but cannot be solely blamed on that.
In a nutshell, there are too few of them. When Harry is introduced to Hogwarts, Gryffindor apparently only has one prefect: Percy. This situation will persist even after Percy is appointed Head Boy. The next year, Harry’s fourth year, we do not hear of any Gryffindor prefect. It is then at the start of Harry’s fifth year that prefects are appointed in fifth year. We learn that there have been a pair of prefects appointed for each of the four houses. We also learn that it is the responsibility of the new fifth year prefects to take care of the newly sorted first year students.
There are a couple of realistic possibilities for Hogwarts.
- Given the small class sizes, there is one pair of prefects per year. If so, it should be exceptionally unusual that they are both in the same house.
- Given the way the magical world clings to tradition, the school may appoint a pair of prefects per house per year no matter how small the class. If so, there should be a new set of fifth year prefects in Harry’s sixth year to take on the responsibility for the first years. Ron and Hermione should no longer be doing this.
- There could be one pair of prefects per house. If so, the head boy and head girl should not count as the prefects for their house, and there should be a new Gryffindor prefect in the third year. We should have also seen a female Gryffindor prefect somewhere in the first four years. Also, if this is true, then new prefects would be appointed whenever the previous ones graduated, not necessarily “in fifth year” even if only fifth through seventh year students are eligible. We should not see all eight prefects being appointed in one year.
It is most likely that there are, in fact, one pair of prefects per house and per year both. The extra prefects are both incredibly absent in doing anything noticeable, represent an easily filled glaring hole in the student body.