It is very harsh to judge a selective school for not getting 100% A-Cs.
Even if there are not academic issues (low level 4 kids massively coached for 11+ who continue to underperform despite good teaching and monitoring), there will always be pastoral, medical and social issues like there are in every school.
As a pp said, divorce, bereavement, abandonment, neglect, illness, self-harm, eating disorders, bullying, illness of a sibling/parent etc all happen to grammar school pupils as well, and can all impact on exam results.
They should be getting close to that magic 100%, yes, but you never know what might have gone on for those 2/3/10 kids who don't get their targeted grades.