It's virtually always the case that there's a higher proportion of A* grades in STEM (Science and Maths) subjects than Humanities, such as English and History, for instance.
With Maths, if you're really good at it and learn the techniques perfectly, then it's much easier to get full marks - the answers to the problems are either right or wrong; there's no dispute.
However, in an English exam, for instance, an essay that one marker may award a B grade to may receive an A grade from someone else. On the rare occasions that someone gets an A* in an essay-based subjects, it's because they've got both a) exceptional ability and b) they've been fortuitous with who's marking the paper (i.e. an examiner that agrees with their viewpoint and likes their style). It's inevitable that there's going to be some bias, as the nature of the Humanities is that there are grey areas and subjectivity.
This is why you can get 100% in a Maths exam for the reasons outlined above, but it's nigh on impossible to obtain the same in the likes of English and History.