That's interesting. The only simple mechanism I can think of for such a big change would be if there were many more children sitting the kent test from private schools than were previously. 2023 entry was before VAT though so not sure why it would change so significantly.
More tutoring in state applicants is also a possibility, as you say. But why such a big change? Again if it had been the VAT year it would possibly be last-minute entries - not giving time for tutoring.
I don't think that state schools teach to the test @OccasionalHope - parents often complain about that!
The other big change since 2019 is variability of teaching between schools during covid. The change isn't the direction I'd expect though, since I'd heard that private schools generally did more than state schools during covid - but must be area-specific, so maybe not in Kent.
There was also demographic-specific variation during covid, eg parents who were furloughed were able to concentrate on education 1:1 with their child. Maybe private school parents are more likely to do the types of jobs which can be done remotely (and also lots of doctors) so not so many were furloughed. But it seems unlikely that this would be persistent enough to make such a big difference 3 years later.
Overall, it would be good to see a FOI request about numbers of children applying (in addition to the percentages who succeed) by actual school, not only school type (not all private schools are the same) and see how it's all changed.
If the numbers don't explain the change (ie if there aren't many more private school candidates than previously) it's perhaps worth a FOI to ask about how the scores are standardised, and also exactly what factors about the candidate are taken into consideration and how.
Some comments I've seen on this board would make it not entirely a surprise if previous school type was taken into consideration. That would obviously be completely unacceptable for a state education which is paid for out of everyone's taxes - especially if it's happened secretly without debate - and should certainly be publicised if it's the case.