Not quite that simple Ferg, your marks for the paper are converted to UMS marks, then its the amount of UMS marks that determine your grade. And each year the amount of raw marks for the paper you need to get 90%(A*), 80%(A), 70%(B) etc changes depending on how hard the paper is. The harder the paper the less raw marks you need to get 90% of the UMS, so one year with an easy paper you might need 95/100 to get 90% of the UMS, the next year could be harder so you could only need 85/100 to get the same UMS mark. It's also a way of controlling how many people get certain grades so when you take an exam your competing against everyone else really. It's a bit complicated but makes sense to make sure that people don't get worse grades because of harder exams.