Well said @izimbra .
"Normal non selective independent schools have a normal mix of children, a large majority of which have SEN, which is why they've been sent there in the first place.
So it's absolutely no surprise that normal DC in state schools can do better"
But the difference is that "normal non selective independent schools" have parents who (i) can afford to pay fees and (ii) are invested and interested enough in their children's education to spend that money on school fees.
Many, many children at comprehensives face huge barriers to learning, first and foremost parents/carers who cannot support them. There are lots of reasons for this, including lack of English, lack of time, the sheer exhaustion that comes with poverty, poor levels of literacy and numeracy etc etc.
My son went to a state primary in SE London and now goes to a state comprehensive in the same area, where there are a high proportion of students on FSM. It is shocking what many students have to deal with.