As said above they are looking for particularly good students and do not want to rule people out unfairly. Also look at the grades of those actually awarded places by the way (which is NOT the same as the minimum grades required) as that is often an awful lot higher.
Oxbridge due to the interview process is looking for someone very interested in the topic etc and very bright and sometimes you find that amongst someone with AAA rather than 3 A stars (having said that my relatives who were recently at Oxbridge all had three A stars and applied after A level grades were out on a second attempt)