Dropped ASs and extra-curriculars won't account for much, however. I'd imagine it would still put the average grades at AAB or similar. That said, this is entirely normal - if BBC is the lowest possible set of grades for somebody accepted, of course the average will be higher! Perhaps it is higher than you expected, but Manchester is a good university, and you'll probably find a fair number of applicants who are very very good - much better than the offer, that is. I mean, you only need to take a look at the grades of successful medics who got into AAA universities to see that nowadays, it's getting the offer that's the tricky part, not meeting it!
All that said, there's no reason why you shouldn't apply if you're predicted to make their offer. Some applicants will be better than you, but that certainly doesn't mean you haven't got a good chance. If BBC is what they've set their offers at, it's because they're happy to take people who are going to achieve those grades. Sure, there'll be many successful applicants who far exceed those grades, but there'll also be a fair few who only just manage them.