I know people who probably deserved to go to a top 10 uni, but not Oxbridge. They are academically competent as far as exams are concerned, but have nothing outstanding beyond that. I know one who got into Cambridge who did not read beyond the syllabus at all (he did not read the books cited in his personal statement), and even often complained about reading challenging texts on the subject. He rarely gave intelligent answers during discussions in class, and he's really quite mediocre (often got confused) when it comes to the more 'advanced' stuff on the subject.
Luck plays a huge role in the interviews. Ultimately, everyone who gets into Cambridge (or Oxford) are competent, undoubtedly. But of course, there are many other people out there who were rejected but arguably deserved the place just as much/more than some of the lucky ones, who were good on display (on paper and during the short interviews), but genuinely and intrinsically- really not good enough.