It looks like some teachers over-egged the scores (what did they have to lose?) and that possibly cause the huge downgrading right keep marks at ‘averaged out’ (so yes, the likes of Eton who got 75% AAs and 93% A-B (2019 figures) won’t exactly need to inflate predictions will they?*
It's not a case of 'over-egging' scores.
The nature of exams means that in any group some will do unexpectedly badly because they panic and don't answer the question or because their life fell apart in the months leading up to the exam, or because they simply didn't revise.
This is one of the reasons that the 11+ was abandoned, and also why generally marks tend to be higher with continuous assessment.
It is impossible for a teacher to fairly predict which pupils will really underperform in exams because of external events. However, any algorithm based on previous performance must take those events into account if its aim is to create a similar range of results.
Of course some pupils do much better than predicted in exams and some pupils don't do well with course work based exams, but its still difficult for a teacher to incorporate in a prediction the kind of event that would turn an A into a D or a C into a U.
I don't know, but would imagine, that pupils who go to private schools are more protected from the kind of events that can cause huge fluctuations between predicted and actual results, and that this is one of the reasons why their predicted grades tend to be more consistent.
the unfairness is increased when a student from a previously poorly performing school isn't allowed to beat the average.