The exams themselves however are an entirely different matter, and I know of several capable students who have had their university prospects diminished by poor performance in the exam room - whether due to anxiety, poor 'exam technique' (less relevant for maths of course), or other factors.
Notably, Cambridge interviews a high proportion of candidates who apply for maths, which presumably enables them to take some of the emphasis away from the exam results on paper, and give much more consideration towards the logical thinking and problem solving skills useful for the Tripos.
I have taught plenty of students who possessed a deep understanding of mathematics but could not perform within the confines of a structured exam. They may be berated for performance or even lack of preparation, but given that problem-solving is the main criteria for admissions this surely reflects the limitation of the measuring tool.
Similarly, many students who breeze through A Level exams may have thought they'd mastered topics like calculus, but it turns out the exams only called on their computational fluency. The moment they are pushed conceptually and asked to formulate their thinking with the rigour that undergraduate mathematics, they come unstuck.
The 'creative' problem I had was this: A mouse walks along an elastic band at 2 meters per minute. The elastic band stretches by 1 kilometer every hour. How far along the band does the mouse get? If you got that right, then they gave you a stretch question where the band doubles in length every hour.
Also how the expansion carries the mouse has to do with its movement. E.g. if it's galloping it would not carry it.
- fundamentally does not grok University-level maths at all;
- gets "ordinary" university maths (ie redbrick and "below"), but would not be able to cope with Oxbridge level;
- Oxbridge level
There are stark step-jumps between these three, and exam results are not a good indicator of which one an individual actually falls into. In particular, I believe you can be a good jobbing middle-level mathematician by learning the relevant stuff, providing you have the basic aptitude for it, but you need a real gift to be able to cope with the top level, and that's something that an exam format cannot elicit.
Speaking personally, I did an interview at Cambridge, which served the excellent function - for both parties - of determining that I was not in the top group.
That's not to say I couldn't have done well on the tests, I just didn't care enough to study hard for them because I had more interesting things taking up my time.
Now, that's not to say there aren't problems with that philosophy too - but it's certainly one reason.
Almost all of those on the Cambridge maths course (which I'm taking to be similar) found A-level maths/further maths very easy. Of course, the admissions process selected for such people, but I think "finds A-level maths content very easy" is a prerequisite for the course.