UMS is dependent on how the rest of the country did. If the exam was easier in one year where lots of people did really well, even if you drop one mark then you might not get the full amount of available UMS. If the exam was relatively difficult then you could drop three marks and get full UMS.
Getting high UMS at GCSE is unimportant anyway and has little utility beyond your own satisfaction. Like fefssdf said, you can't count on consistently getting full UMS at A Level.
I'm pretty sure it's the same for all the exam boards; however, a couple 'freak' papers occasionally buck the trend. According to my maths teacher, there was a FP1 paper (which I assume you won't be doing whatever happens) which a 80 UMS was around 65%.
You shouldn't get deterred from maths just because the UMS system operates this way.