Some colleges are more over-subscribed than others, so applicants are spread out to ensure that no college is more or less competitive than any other. It's not a reflection of the candidate's ability, it's just done to ensure that the process is fair. This 'spreading out' process happens again after A level results day, where applicants with open offers are assigned to colleges where some of the students with offers from those colleges didn't make the grades, or withdrew their applications. They don't make loads of open offers, though, so some years may still have fewer students than others, but on the whole, the number of students for each subject at each college just depends on how big the college is.