I want to note that as a current Oxbridge academic, I don’t actually agree with this that much — first of all, not as many students apply with 10A equivalent and 4 A predicted as you suggest: they are most definitely in a (small) minority.
(The new numeric GCSEs also make it unlikely that many candidates will get straight 9s, as the 8/9 is meant to cover a range of marks across the old A*.)
It’s actually reasonably rare to get candidates with all 9s. A mixture of 7s, 8s and 9s at GCSE is more usual. And many of the students we accept have a mixture of GCSE grades — sone of my highest achieving students didn’t have stellar GCSEs on original application. Some kids who are suitable for the course might be all-rounders with high GCSEs across the board; others have a mixture, with attainment spikes in the subjects they’re especially good at: that’s pretty normal.
As long as a student is predicted to meet the “standard offer” (AAA/AAA) at A-level, we will normally interview them — and not all of the ones we admit get straight A across the board by any means.
I have to say that we tend to look for slightly higher exam achievements across the board from candidates at the very good grammar or private schools; we’re less bothered about the results profile from schools with a poorer overall teaching record.
I actually am pretty confident that we tend to mostly get the right people. I’d say that in every admissions round, we’re pretty confident that 60-80% of the candidates we admit are well suited to the course. The remaining percentage is a bit less easy to judge: on that borderline, I do think we probably accept the occasional person who isn’t quite suited, and always end up having to reject a fair few who we could also have taken.
(Those candidates often have an opportunity to get picked up in the winter pool (Cambridge) or by the second/third choice college (Oxford). There’s a smaller pool/adjustment in the summer when results come out, for those who narrowly miss their offers, too.)
Of the students we do take, there’s quite a variation in A-level subjects/GCSE profiles/backgrounds. Sometimes you get an applicant who doesn’t have a great exam record, might be at a not great school, but they have a really natural intuitive feel for the discipline, and do you want to take them. (This is more common in arts/humanities than in sciences: they tend to rely much more on exam scores ok the sciences.) Sometimes - in fact quite often - there are candidates with eleven 9s at GCSE, fab A-level predictions from top public schools, and you interview and they just don’t have a feel for the subject at all: they aren’t really a natural or they can’t really develop their thinking, and you can often clearly see the exam record is evidence of good teaching and spoon-feeding, and not natural ability.
In the middle, there are lots of kids with good but not perfect grades, decent written work, decent test results, and who put in good interview performances, and we’re looking to take those who have shown across the board evidence that they are bright, have a feel for the subject, can think on their feet, can develop a line of thinking, and are able to show enthusiasm and interest in a range of issues that are part of studying the subject at university level.
And then we make a list ranking our top ones, and typically the Admissions or Senior Tutor cuts off our list at the number of places we’re allowed that year; and there are always some who fall below that line who would probably have done well too, and that we would have loved to take if we had more staff and more places.
Overall, though, I tend to be pretty confident in the ones we do take. (They do seem to do pretty well, if I say so myself; or maybe I’m just really good at picking them 😂 we regularly come top of the university in my subject!)
However, there are many other great departments and universities, too, where students will also have great teaching and a great time doing their degree. So I often don’t agonise too much about the students we don’t take, because they are likely to be a gift for my colleagues in another good department in another good university, and do very well wherever they go!