From my understanding if in a normal year 5% get a 9, 10% 8, 20% 7 etc and last year 10% got a 9, 20% got an 8, and 40% got a 7 then this year the grade boundaries would be set so 7.5% gets a 9, 15% will get a 8 and 30% will get a 7 etc. . The actual grade boundaries won't be determined until the papers are marked. Ignore the percentages I used, they are not accurate just there for illustration and easy maths.