One of my ex teachers worked for the JMB (exam paper authors) as an exam marker. He said, at the time, that the top x number of students got an A, the next x number got B and so on down to E. So it wasn’t down to what percentage you got in an exam, it was where you ranked alongside your peer group doing the same paper, and it was done this way to even out different difficulties of papers between years. That means every year should have roughly the same number of people getting each grade, but clearly the system changed somewhere (I’m guessing when GCSEs came in) so that everyone gets an A or A* or A** or 1 or whatever they use now.