well it tends to be that if you can understand one science/maths, you can understand the others, whereas if you're really good at art, you're not necessarily good at english. it's not that there are more A grades available for the sciences, it's probably that those who are capable of getting an A in science will get similar grades in other sciences, whereas people who do say art, drama and tech have more variety in their subjects, and although they may be very good at one or two, they may not be great at the other.
it could also be that it tends to be the smartest people do the sciences/maths, and the people who are less smart do art etc
btw, yes, i AM doing maths, biology, physics and spanish (for a bit of variety)