I think it is true that natural ability plays a part, as does having a good memory. Memory is where i fall down greatly, which is why my performance on synoptic exams is usualy worse than on just normal moduler ones. I think that a lot of the time grades are dependant upon the marker you get, and it worries me greatly. If you get a harsh marker, on a paper out of say 75 where + or - 6 or 7 marks can make all the difference having a nice or a nasty marker could make or break it.
Of course there is no way to avoid this, but i certainly do get scepticle sometimes. There's a guy in my biology class who constantly got A's in the assessments and he is easily the best biologist in the class, and he got a B in january, only marginally better than my B and i am terrible at biology, usualy got C's in the assessments. And on the other extreme there was a guy who always got E's and D's on assessments and didnt know much about biology, who got an A in the test.
Could just have been luck, but i think the marker has a lot to do with it. Especially on very precise mark scheme's like AqA ones. Some examiners will stick very closely and not award certain marks, while others will see the student simply missed one of the words, or worded it another way, and will award the marks.