Just was playing around with this converter, to see what sort of raw marks you needed in order to get a certain UMS mark and I was shocked to see how much they vary from one exam series to the next. For example Core 4 in January 2011, you needed 61 raw marks to get 80 UMS, 68 raw marks to get 90 UMS and full marks to get 100 UMS. In contrast the same unit Core 4 in June 2011 required 51 raw marks to get 80 UMS, 58 raw marks to get 90 UMS and just 65 raw marks to get 100 UMS. I mean really do the exam papers differ that much for the grade boundaries to be so different?
You have to think about how much the grade boundaries fluctuate each year isn't purely to do with the exam paper, it includes the intelligence of the candidates sitting the paper.
Apparently, according to my lecturers, exam boards like AQA have to maintain a certain amount of 'high' grades and a certain amount of 'low' grades so that it doesn't appear their courses are too hard or too easy; they supposedly push up their grade boundaries if lots of people did well, therefore reducing the amount of high grades actually attained and vice versa.