Technically, some people could get a B at AS level and can get an A* at A2 but not an A. As the requirement is just an A overall but an average of 90+ UMS in C3 and C4 the people that only just manage the A had to get 90 in C3 and C4 to do so , so they get an A*. So yes you can get an A* in A2 without having got it (above 90) at AS