@QuebecBagnet because the teacher grades were still meant to be based on evidence. And if the children in private schools got a better education than in state during Covid (generalising, obviously), then they're likely to have produced more evidence of good grades.
It's also the case that private schools often had more resource available to assess that evidence, rather than spending less time on it. I have a friend in a senior role at a private school, and they sweated blood, swear and tears to assess evidence and try to ensure optimistic but fair grades (which is precisely what the government told them to do) - I mean endless, endless hours of work.
And just from this thread, there are plenty of private schools whose grades are higher this year than last, too.
(NB your anecdote about the pupil with the U - that was the government algorithm, which applied to all schools, not just state. Those kinds of anomalies are the reason they scrapped the algorithm at the last minute.)