Regarding the % of 2.1 or above rite, the maths dept is trying to keep standards I guess. The college would want they depts to give more 2.1 or above because this is counted in the newspaper rankings and it would help swing whether Imperial is 3rd or 4th.
I have a friend who is doing law at LSE. According to him, for his current year, about 23% got 1st, while 70% got a 2.1!!! Hence LSE is trying to keep standard by giving few 1st, but then trying to satisfy the newspapers by giving 2.1s like candies.