I have a Scientist daughter and her peers went through this a couple of years ago. I think Yellowtip's advice is very wise. Your daughter sounds like an amazing candidate and should get in but some of the ablest candidates don't , and some surprising ones do. She will however get in to an excellent course elsewhere, wherever DD's Science geek friends ended up they are loving it, are being challenged and inspired, and being taught by exciting people at the frontiers of Science ( Hadron Collider Physicists at Durham, Steve Jones at UCL etc etc) A lot of very able Scientists go for Medicine, or Cambridge because it has a reputation for being the Science Oxbridge and the Natural Sciences course is highly regarded if they are going into research (cross disciplinary knowledge is of benefit in some cutting edge research). It certainly seemed marginally easier to get on to some of the specialist courses at Oxford (certainly Biology) but clearly if someone is passionate about specialising they will choose Oxford rather than Cambridge. Also bear in mind that some non Oxbridge Science courses are actually more difficult to get on to, depending on specialisms and their place in world research rankings. I gather the Durham Chemistry course is notorious for rejecting applicants who get into Oxbridge (including one of DDs peers predicted AAAA) so might be an unwise number 2 for all but the very cleverest. And certainly Cambridge seem to have frequently added to feedback that although they had been unable to offer a place at undergraduate level they advised they reapply at Master's level!
DD's school advised that there are two big sources of uncertainty. Firstly that you don't know the quality and quantity of candidates applying to a particular course and college in any given year. They publish admissions stats but these just encourage strategic applications, so that less popular colleges and courses become popular the next year. Though Cambridge have a formal pool and Oxford an informal one, a good cadidate seems to have less chance of being picked up if they were just unlucky enough to be applying to a very popular course at a popular college. Secondly you are interviewed by the tutors who will teach you and though they will try to be objective inevitably they are going to be swayed if someone is clearly going to be a pleasure for them to teach. Although your daughter should stick to her guns in terms of what interests her, that is where she is going to demonstrate passion for the subject, it might be worth a bit of reading up on, and bearing in mind, the interviewers area of interest. It might help establish a rapour and if nothing else it might be what triggers an off piste question! I know of someone who was interviewed by a plant biologist though their area of interest was molecular biology. They got tripped up because they were asked to talk about a stag beatle, they went into all sorts of details about the advanced biochemistry and various qualities at a molecular level of beatles (sorry this is probably rubbish in Biology terms but that sort of thing anyway!!!). They were so keen to show off and got so stuck in their train of thought it didn't occurr to them until after that when the interviewer kept asking if there was anything else, that what he might have been after was more wide ranging and in his area, rather than more detailed, things they assumed didn't need saying, for instance that it was called a stag beatle, it's position in the food chain, role in the environment, endangered status , especially since the candidate has included in his application that he volunteered in a nature reserve (and had done studies of stag beatle numbers!) It was mentioned on the feedback .
I hope this helps. Good luck to your daughter.
And above all do warn her this is just the start of it. The step up to university level science is a big one, they have a lot of contact and lab hours, a high workload and are really challenged. Even some of DDs peers who achieved 4 A*s have found themselves struggling, and even retaking years, and not because they were a fixture in the bar. Some of the advanced concepts they are studying are just very difficult to understand.