[pmyteh]

Agreed, but there are differences between different classes too: the students vary somewhat in intrinsic quality and motivation, and in those years I've done a particularly good job of teaching and motivation the results can be better overall because the students are putting more effort in.

Similarly the other way. There are classes I've given no first class (~A) marks in, because nobody earned one. Still a curve in practice, but with the top at an upper second.