I add that I have been told that some people are so dazzling on paper that they can survive a bad interview and still obtain an offer. Those who look OK on paper but freeze up to an extent greater than nerves can reasonably excuse, or don't keep to the point, or seem a bit rigid in their thinking, may get the chop.
I think that the tutors try super hard to be fair, but they are human, and they have to face teaching a person for three or four years, so try not to be boring. Being boring is the greatest sin in Oxford, a place which prides itself on wit. This does not mean that you have to be Blackadder. You do not have to put on a performance. Just be that thoughtful and intellectually curious late teenager who can see beyond the narrow compass of the A level syllabus.