It's not very surprising.
Imagine you're a teacher. You've got some kids you are pretty sure will get 7, some you are pretty sure will get a 6, and some who you're almost certain will get 6 or 7 but it could go either way on the day. I reckon when I was teaching it would probably be half where I was reasonably sure of the grade they'd get and half who would be on a borderline. Add in that you don't know how well they'll use the last couple of months of study time, and it's less predictable than usual.
So, if your kid is one of those 6/7 borderliners, what would you like the teacher to put down? 7, obviously; 6 would make it unlikely that they would get a 7. The teacher knows that some of those 6/7s will come out as 6s when they do the statistical adjustments, but not how many. So they might as well put them all down as 7. They will probably have agonised about the ranking, knowing that the bottom few are more or less certain to lose out. I expect many have gone for a formula (eg mock score with adjustment for work seen since then) to keep it objective - that will inevitably work better for some than others.
So it's entirely plausible that over half of pupils are in that "borderline" position where the teacher has put the higher of two possible grades, and you might reasonably expect at least half of them to get the lower one.
In the end of the day, there will be a bit of randomness at the borderlines - but then there always is, because everyone knows that there's a bit of luck of the draw in the questions on the exam.
I suspect most grades will not move more than one notch, so all those 6/7 borderliners will get a 6 or a 7.
Yes, it's going to be frustrating, particularly for those on the more crucial borderlines.