I know its stupid Itchy, but I know at least one example of this DEFINATLEY happening. (well its bristol not durham but very similar quality university, similar arguments apply etc.)
A guy on my year, (he is on tsr -- username j2k so this isnt some made up story) got rejected from bristol for economics. He has 10 A*, 5 As at AS. A superb personal statement, genuinley intrested in economics, he has everything. (Got into Cambridge, in our school with 22 oxbridge offers, i would say he is 1st or 2nd, so basically he is very very very good.)
My explanation why universities do this --- if they figure the chances of people accepting their offer is 0, (which it indeed was in this case) then it looks better for their statistics if they reject someone rather than them rejecting him.
I know its a minor thing, I know its silly, if i were the admissions officer at bristol, I certainly not reject him. But it does happen.
Why else would he get rejected?