Keep in mind that the rating here is mostly about the student's opinion of the university, especially in terms of the teaching. And a dirty little secret of oxbridge I feel is that the teaching isn't particularly great. It's not bad, to be clear, and supervisions are something that you don't really get elsewhere, but it's very much a 'chuck people into the very deep end and let them sink-or-swim' approach with the sheer amount of material that they expect you to get through in a pretty short amount of time. The reputation they have is because a) they can afford to be (and are fairly good at) being highly selective in admitting those who can cope with this, b) if you do manage to swim, you've shown a higher level of aptitude for the subject than other universities, and c) because of said selectivity, you'll tend to be around other students who are able to support you. The actual level of support and teaching from the university itself is not as top-class.