Bucks exam - Standardisation

[southbucks3]

If a 121 qualifying score is supposed to qualify about 30% of the children in Bucks, how does that fit with the explanation above? Under the 'normal' standardisation model, a score of 120+ would only be achieved by about the top 10% or so, wouldn't it?

A normal standardised score to qualify the top 30% of candidates would be around 110-112, wouldn't it? Anyone understand how that works?

Standardisation against a national cohort, perhaps? Is your average Bucks child brighter than the average average child?

Much as I would love to think that the bucks blood runs with academic fervour, the cohort sitting the test is not equal to a national representation of the same quantity of children chosen at random. Several parents opt out, which takes away from (mostly) the bottom end and a ludicrously huge number of late, opt in from out of catchment, which adds to the top end. Also the top 40% are more likely to be pushed, tutored and trained so again the standard is raised further. Add to that the vast number of parents who move here due to the grammars (even if the parents are misconstrued the chances are their children will be brighter or more tutored than average) add the wealth and high level of privately funded primary education in the Chilterns and you have a skewed statistic with a high average test result and an even higher standardised pass mark.
The best indication of a normal (equivalent to national) cohorts chance of passing is aylesbury vale pass rate, which before review or appeal was 17% last year not including those who did not enter.
The "too 30% of bucks children" statistic hasn't in reality existed for over twenty years.