I think generally teachers over grade, to be honest, and certainly they will have almost sure to have done this time. I was disappointed with my own A level grades, which I felt I had worked hard for, but I obviously wasn't as good as I though I was - that was in the 60s.
Grades have been too high for years, in my opinion, since the comprehensive system overtook us, which points to a lot of the work being too easy. If your children had actually taken the exams, their grades may have been worse and they can retake later in the year to prove their real competence.
They would have started taking exams in early May so only missed a few actually teaching weeks, many of those would have been revision times. I wonder how many teenagers actually knuckled down in lockdown to continue full time home study. Not many, I would guess.
No employer has ever actually asked me for my grades in 45+ years of working! Mumsnetters have to be enraged about everything.