Because it seems to vary so much depending on school as to what grade they award to what percentage, e.g. 60% may be a C to one school, and to another an A!!! I teach in a prep school (first year after moving from a state school), for all the children whose CE papers we mark the head decides on levels after seeing our percentages, he has a rough idea of how similar schools would grade. The person I took over from, who was Head of Maths as well as the academic deputy head suggested to me that in the Level 1 Maths paper you bump up the percentages by 5%, e.g. if 50% was a C then it becomes 55% to get a C, and on the Level 3 to reduce by 5%, so if 75% was an A make it 70%.