Bear in mind on the student room that people with more 'average' (even if still good) grades are going to be less likely to post them online than the multiple A* club, because yah know, if you've got it, flaunt it. On lots of AAB courses and stuff on all the applicants threads all the users are like 'predicted A*A*A*' etc. when if everyone on the course got those kind of grades, the entry requirements would reflect it.
The whole idea of the A* was to separate the super good from the good. It would be utterly pointless if everyone got straight A*s. I think, though I can't be sure, around 10% of all Alevels are A*s, meaning that roughly one in three people will get one A*. (Obviously it's not as simple as that: There'll be schools at which no one gets A*s and others have most of their students achieving straight As and A*s, but it should give you an idea)
I don't have any evidence to prove it but I'd be willing to bet there's more straight A*s in maths/science students than in any other area. Moat people I know who have got 3, 4 or 5 A*s have done Maths, Further Maths, Chemistry, Physics and Biology. It's not that these subjects are easier, it's just that some people have a mind for it which means at least at A level it's kinda easy. I only know two people predicted A*A*A* for none-mathsy/sciencey subjects, whereas I can think of several who are applying to medicine or maths or physics et al.