Different exam boards is not the reason -- a single exam board can (and does) offer the flexibility of topic in areas the National Curriculum allows this, and sticks to what's prescribed elsewhere.

I think if you feel time is wasted for something like history, geography etc that people have uneven knowledge of, that's because they've failed to learn the techniques or fundamentals, and can only apply what they've learned to a single topic. There's little difference to everyone having to study WWII at GCSE, then only knowing what (not why) at A-level.

Science and maths have a lot less flexibility.