When I couldn't sleep last night (how many days is it until Thursday??) I realised that the major problem can basically be summed up as (apologies if everyone else is way ahead of me on this!)
If 1000 students sit exams, there might be 300 who are genuinely borderline between 2 grades so the exam could go either way. And there might be another 100 who have bad days and do slightly or significantly worse than expected.
Teachers don't know which of the 300 borderline students will go which way, so they've given them all the benefit of the doubt and put them up. The algorithm will allocate these 300 to "up" or "down" but actually it has no way of knowing either.
Teachers also don't know which 100 of their students will have a nightmare on the day, so they will give them all their "normal" predicted grade. The algorithm will pick 100 students and give them a grade that was different to what they were expecting, but actually it has no way of knowing that they would have had a bad day either.
It's one thing to be borderline and not do as well on an exam so that you get the lower grade.
It's one thing to have a bad day and get a worse grade than expected because you messed up.
But being told that you cannot sit an exam and that you'll get a worse grade than you should have done, because a machine has decreed thus, based on a whole bunch of things that are entirely out of your control, is not, and never could be fair.
Yes, there will be some (hopefully not many) maverick teachers who have way overpredicted.
But in general the above is why the grades this year are, and indeed should be, because teachers are not clairvoyant, higher than "normal" years.