OK basically in my opinion, actually not in my opinion, it is a fact that it is by far easier to get an 'A' in a science subject than an 'A' in a humanity subject. Firstly science subjects is just cramming in facts and figures and just using that in the exam, especially in something like maths, whereas in humanities there is that element but just cramming in your text will not be enough whereas for a science subject it will be. Secondly Humanity subjects are subjective, there usually is no real way to distinguish candidates as something they write could be wrong in someones eyes and right in someone elses eyes, whereas sciences its just one right answer.