I think it's quite interesting. My experience was maths.
I was not many years down into the line of GCSEs and could (good mathematician) do o-level maths papers-in fact I rather enjoyed them.
There was also a noticeable step up in difficulty on A level papers when you went onto the ones who would have done O-levels, but I could still do them.
However dbr was 5 years younger than me, and was also considered good at maths (As in maths/further-before A* came in) and he not only couldn't do the O-level papers, and the equivalent A-level ones , he couldn't even get an okay mark on the A-level papers from my year. It wasn't that he hadn't covered the subjects, he just hadn't covered them in enough depth.
My memory of my observation on the paper from that time was that they totally spoon fed the questions.
So whereas my paper would say (eg). Integrate f(x) wrt x. from x=3 to x = 2 (6 marks)
His would say:
a) prove that f(x)= (something that made it easy to integrate) (2 marks)
b) show that integrate f(x) = integrate (something that makes it easy to integrate) (1 mark)
c) Show that integrate f(x) wrt x = (answer in x) (2 marks)
d) What is integrating f(x) wrt x from x = 2 to x = 3 (1 mark)
So although on the face of it, the same work is done, they tell you exactly how to do it (for the non mathematical there are several methods for integration and in most cases there's only one or two that are best, so choosing the right method is part of showing your understanding how it works)
You are given the answer for most of it. It's much easier working towards a known answer. You're much more likely to end up at it!
And the other thing. Even if you don't understand one part, you can just pick it up from part way through.
I've picked this example because I had an experience with someone in my year who was doing maths, he was B/C borderline, so quite reasonable, but not at top.
I came across him in tears over a exercise of integration. He'd been fine while doing single methods but was all at sea having been given a mixed exercise. As I was talking through with him, I found that if you gave him the method to do the question he was totally correct. ie he'd have been getting top marks on the later papers that dbro did. Thing was he could work through a method fine, but he didn't have the understanding why that method worked.
I spent ages talking each one through with him, and he understood most of the methods by the exam time, so as long as it was one of those he'd be fine!
My English however was the easiest ever. We were 100% coursework and did 10 essays total over the 2 GCSE years for language and lit. They all counted towards language, and about half counted towards lit. Was ridiculously easy and stress free.
Also the chemistry first GCSE papers I remember used to start with multi choice. The first question was something along the lines of. Is this a picture of a) Bunsen Burner, b) test tube, c) safety goggles d) next door neighbour's pet goldfish.
They'd stopped that by the time I got there, but I think the idea was to start off with an easy one all could get, but even so!