Around 70% of students at Oxbridge have achieved 3 A star at A-level. Another 20% have achieved 2 A stars and an A and then about 8% have achieved 'only' one A star and two A grades. A tiny percentage have AAA and less than 0.5% will have a B (there is likely to be highly mitigating / contextual circumstances for AAA or a single B).
For all those who get in with two or three A star grades, on average 5 times that number are rejected who will go on to get those grades. So grades alone are a guarantee of nothing.
At Cambridge, 24,000 applicants for just over 3,000 undergrad places and Oxford is the same.
Offer rates vary between courses - eg for Classics or MML it might be 1 in 3 chance of an offer, but for other courses it might be 1 in 15. The average ration of offers to applicants is 1 in 6.
The interviews and aptitude texts are an important part of selection. There is a supplementary personal statement for Cambridge.
Basically, loyd if very able students with top grades are rejected every year. They will generally go to Imperial, UCL, LSE, Durham or St Andrews. Failing that, Bristol, Warwick, Edinburgh, Bath or Manchester..