Roll a dice 30 times. How many sixes did you get?
www.random.org/dice/?num=30
I tried ten times:
5, 4, 6, 5, 6, 2, 7, 7, 4, 4.
Now imagine that's a comprehensive sixth form sitting an A Level. Even if we know the school's results for years and years, and we know 1 in 6 get As, then in any given year there are likely (99%) to be between 2 and 8 As in that class.
If you want to award the same number of As as last year, you're going to need to hand out 5 As to that school. But this year just by randomness maybe only 2 deserved it. Lucky kids. Or maybe 8 deserved it. Ouch.
Now imagine if that was a smaller class only 15, and normally on 1 in 15 get As. If student performance is randomly distributed then there's a 99% chance of 0, 1, 2 or 3 As. So you can have 3 A*s in a class that normally gets one. Not every school would be like that. But some classes in some schools WILL be in that position. And it's in fact NOT randomly distributed. Some schools will have good intake years. Maybe a genius physicist's twin children joined the school and are sitting A Level Physics. So the exam results are actually less likely to follow the past distribution than even random dice would be. It's even less predictable than that!
Even an elite school where 40% get As, is going to have a considerable variance from year-to-year even in large subjects like maths. In smaller subjects with 10 students per year, the model proposed to award grades according to the past 3 years' results. In the past my son's school got for music 100% As some years, and in other a mixture of A, A, B, C. Unlucky for you if you were in the all A cohort.
It's astonishing how anyone with even a GCSE in Maths could think any of this could make sense. It's the most basic principle of variance.
Surely they couldn't really be so incredibly stupid?! Was it all a trick so that those given unfair grades by their teachers would not complain about that, because well at least you weren't awarded a D by ERNIE.