Dd1's first exam the teacher reckoned the marks were about 10 (RAD) below what she would expect. This was shown to be pretty accurate the next time they did exams really.
One time though one local group had a slightly weird one where they were a little down on distinctions compared to normal, but not more than a couple. Some of the distinctions were a slight surprise, and some were a surprise they didn't get one. But there was a noticable gap in marks. Distinctions were over 75, and almost all the distinctions got 75 or 76. But then the next marks (Merit) the highest was 69, so 6 mark gap.
My theory was that the examiner was a tough marker, but then looked at the results before she handed them over, and realised that the lack of distinctions would be noticed. So she upped those who had 70 and over to be in the distinction level, was still a bit low on numbers and so upped a couple who were on the 67/68/69 level, but not them all, hence the slight random look.
At the dc's dance school it is unusual to get below a merit too, however, they tend to do the grades about a year later in age compared to other local groups who have a bigger range of marks.
I also think the examiner is inclined to choose what mark they think then share it out between the different catagories, as people are always pretty consistant what they get. Usually mostly one mark (out of 10) for all the catagories.
But the same is true of music exams-when I was a child I had an examiner one year who got struck off the board for hardly passing anyone in a year. I got 100. (pass mark) 