>You could have an A one year and a C the following year with the same paper.
I agree that raw marks are converted into a UMS mark and then boundaries are set, but I wouldn't agree that a C one year could be an A the next on the same paper.
For a start there are two tiers of paper- Foundation and Higher - and on the first the highest grade possible is C whereas an A can only be achieved on the Higher paper.
Certainly as far as Maths is concerned there is a big difference between a C and an A. My Year 11 students have taken about 10 past papers over the last 2 months and their marks are very consistent indeed from paper to paper, showing a modest improvement of up to one grade.