This really does seem very unfair. I went to a middle-of-the-road comprehensive, and in one particular subject our entire (small) class happened to be talented

This really does seem very unfair. I went to a middle-of-the-road comprehensive, and in one particular subject our entire (small) class happened to be talented - all of us but one got A*s. My understanding is that this wouldn’t be possible under this algorithm. On the other hand, I’m struggling to think of a system which would be better, which also preserves the integrity of A-levels as a qualification.