I'd say it's mostly the pressure to do well, although I would've also voted for the inaccuracy of the marking. I'm one of those unfortunate people that don't deal with exam stress particularly well - I excel at classroom assessments and at coursework, but struggle to use my full mental capacity under the pressure of timed conditions in the exam room. Some people seem to cope with stress so much better, and therefore often get superior grades despite being obviously less intelligent.
As for marking inaccuracy, I have genuinely seen people go from E to an A - this happened to my brother for his WJEC PY1 exam paper, where the examiner was highly incompetent, missing out at least 3 pages worth of questions.
I also would've voted under the "Other" category, since I'd like to state how frustrating it is when you miss your required grades by just a few marks - is it fair that someone got into their desired university and you don't simply because you missed an A by only two UMS points?