If a candidate has the academic ability to cope with the course at Oxford (or Cambridge) for that matter, then without doubt that person should be accepted, if there are places available - if there are 100 places but 500 applicants, the uni has an obligation to pick the 100 most academically able.
However, too many people associate academic capability with exam results, but Oxford (I presume the same with Cambridge) do not, always, but merely use A level results as a guide.
If a pupil gets ABB at a poorly-funded sixth form college where the average results are DDD, then it proves that the candidate can perform above the provisions of his environment - hence it may be an inclination towards a sufficiently academically able student for an Oxbridge course.
I know for a fact that the admissions tutors at some colleges in Oxford do employ this type of reasoning, and so often pupils from state schools with poor results will be given a lower offer - I wholeheartedly support such a system and do not consider it unfair (and, for reference, I come to the end of my A-levels next week, at a private school, and gained a place at Oxford which will start in October).
As for whether it should be routine that state school applicants receive lower offers, of course not - each case should be considered on its own merit.
As for international students (^^^) I think there are often other issues - it may be that some international students apply without proof of funding (after all, uni can be expensive for non-UK citizens, and incredibly so for non-EU citizens). Hence refusal would be automatic even if the candidate was the next Stephen Hawking!