Also consider this:
In e.g. 2010, a school has great results and a great (outstanding) recent OFSTED.
Parents who really care about results (more than about pastoral stuff) jump through all kinds of hoops to get their children into this school.
A couple of years later, the head teacher leaves and a new head takes over.
The new head may be really clueless, but OFSTED never comes back, so the school retains its 'outstanding' (and still has it in 2020).
Meanwhile there are a lot of staff changes and the quality of teaching goes down under the new leadership.
But the children taking SATS in the new head's first two or three years have profited from many years under the old head, so SATS results stay very high.
Every year, the children who start in reception are children whose parents really care about great results.
Another two years later, maybe in 2016, parents of the children who joined after the 2010 great results and OFSTED, and are now in Y5 or so, realise that their children really aren't where they expected them to be. The school is not delivering. The parents (who chose this school because they care, a lot, about results) send their kids to private tutoring to fix this.
Because of all the private tutoring going on, the results remain high. (In our school for instance, up to 95% of Y5 and Y6 children attend private tutoring, at least once per week...)
The next cohort of parents (who chose this school because of the results) realise that the school is not delivering. They send their kids to private tutoring.
The results stay high, and more parents who care mostly about results, choose this school.
Etcetera etcetera...
Great results can genuinely be a self-fulfilling prophecy that has nothing to do with the quality of teaching, and everything with the fact that people who chose a school for its great results, will make sure that their children actually do achieve good results, even if they have to do this by paying for private tutoring.
There are of course also high-achieving schools that genuinely just have the right culture and attitude and just get things right. But in absence of any kind of recent OFSTED, I wouldn't trust the results one little bit - at least not if the school has a relatively affluent intake. Chances are that the school is simply coasting, relying on its intake (involved, interested parents, and children who will be privately tutored). A school that gets great results from a relatively deprived intake, that's a different story altogether!