Worth noting that written exams do not feature at all in research. There's a recognition at this level that mathematical understanding can not be captured through blunt testing instruments.
I don't recall ever sitting an exam with 10 questions where if you got 2 of them out you'd come first. You'd mark such a "hard" paper based on "this is a reasonable avenue of exploration", "this is a good idea that won't work", "this approach will prove ultimately futile but given the student hasn't seen anything like it before it's definitely worth some points."
Why are you trying to achieve just that in an interview?
Research isn't done in interviews any more than research is done by sitting exams. However sitting down and exploring ideas with a pen and paper by yourself is likely closer than "creating an impression" in an interview. An interview where you, the interviewers, are saddled with the irksomeness of having to roughly account and adjust for your bias as much as you can be aware of it (does she really look like a mathematician? I didn't like sportsman Ben's approach as much as bespectacled whoever - is the impression based on the maths alone or is the form of it's presentation fairly important).
The huge advantage of grading an exam is you can exclude all of that gumf we all carry (with differing expressions of it) by simply not knowing who wrote it. Not their name or anything.
The other big win is that if you publish the blessed thing you can make such exams more normal. Budding mathematicians might practise such skills and get better at them, younger.
As it is you're selecting for the children of mathematicians who get such practise at home and excluding the very smart who honed their exam skills alone because that's all they encountered.
I also got very flattering scores on mathematics exams but I wouldn't mistake that for anything beyond my ability to act the student performing seal at that particular game. Any exam where it is remotely possible to get 100% does not test anything like creativity - creativity is the thing that is most important, no?