FWIW and I am no expert on this, I found the Ofsted reports that I looked at tended to mark the schools with difficult intakes much higher than schools with easier intakes.
My sisters' children went to a very challenging school for a term while they were in the process of moving. A majority of the children didn't have English as a first language so obviously they were well behind in literacy etc.
You wouldn't immediately be aware of this from the narrative of the Ofsted report which was extremely positive about the school and especially the teaching (which I understand was indeed excellent in the circs) but the fact was that the outcomes were not great because the starting point was so low.