The boundaries are rough guides as they are changed once the exams are marked. Mr/Mrs Unregistered said:
Boundaries are calculated at the end of the marking process in accordance with a normal distribution curve to ensure that (to the left of the curve), few people get E's, (to the right of the curve) that few people get A's, and the middle of the curve (where most people will fall with a grade C).
I hope that helps. Good luck!