Oh dear god It has everything to do with UMS. Naturally the difficulty of papers fluctuates between years, conversion to UMS is used to try and ensure different year groups have ~even spread of marks.
If people did badly thats their concern, conversion to UMS isnt in place to subsidise poor performance.
Thousands of people take the test, and they perform differently. Do they look at all the papers in the world then decide?
How grade boundaries are fixed into their raw marks is not a transparent process. It seems to take place after all the scripts are marked, and that includes the whole cohort, including overseas students taking the exam.