Explain secondary school data to me please!

[BigLittleRedOne]

I've been looking at the Progress 8 scores for our local secondaries, as well as the Prior Attainment figures. Considering none of the schools are selective ( some do banding tests but apparently take an equal proportion from each band...), why is there such a difference between schools in terms of the low, middle and high prior attainment scores?
And surely this distorts the Progress 8 figure and it's not a fair comparison?

Without knowing the schools it is impossible to comment on the reason for the discrepancy really.

Progress 8 is a measure of progress not attainment, the starting point doesn't matter, it's how much value the school has added to the children's GCSEs in comparison to children with similar prior attainment.

There will be no P8 data for the next 2 years due to Covid, as Y10 and Y11 didn't take KS2 tests.