They don't do maths at school AFAICS, the sort of stuff we did at infants school and junior school is not in the lower GCSE maths at all. If you saw the TV program about the kids going to a 1950s school you would understand, most of the GCSE kids struggled with the 11+ paper used to select 11 year olds, a few didn't so some must learn something different.
Whatever people say, a GCSE C grade is not the equivalent of an O level or even a CSE grade 1 as they were. A GCSE A grade might be but, you can't tell as they are too easy to get.
Its the same at A level, a grade A may be good, but you can't tell. The spread between the bottom of the A grade and the top is far to much.
More and more people want the actual mark and don't care what the grade is these days, the grades are meaningless.
Some run their own tests, including all government departments ( I know as I had to take them even though I have a degree).