Raw mark to UMS on papers are different, to compensate for difficulty. For example You could get 70/100 in one paper, and 60/100 in another, but they could both translate to 72 UMS due to the latter being more difficult.
Always 80% overall UMS over AS and A2 to achieve an A
You need at least 80% UMS at AS, (A), and then at least 90% UMS at A2 to achieve an A*.
It works exactly like this. 80 AS 90 A2. If you were to get 100% UMS at AS, but 89% at A2, that would translate to an A, simply, you need an A at AS to qualify for the A*, and then 90% at A2 to actually get it.
Different exam boards use different UMS totals. OCR/AQA tend to use 600 Total (300 AS, 300 A2), whereas edexcel for example tend to use 400 Total (200 AS, 200 A2)