I was using the top score of 50% to make a point, and that is: the pass mark (121 in Bucks) equate to a %age in the VR test that would vary from year to year, dependent on difficulty of the papers, the calibre of candidates, etc. If we had an extremely tough paper and all applicants happened to intellectually-challenged, then the %age required to 'pass' would drop to ensure that sufficient of them passed to fill the places available.
All else being equal, if one county has just one grammar school, say with 150 places, the the chances are that the top 150 kids, say 5%, out of the whole county would pass and be allocated a grammar place. In Bucks, we have so many places that 30% of the kids pass and get in. Taking this to its logical conclusion, if a county has so many places that it could accommodate all but, say 5%, of its kids then all kids would pass except the bottom 5%.
As for the 86%, I am not sure what %age equates to 121, but I would be surprised if thats high, given the results tend follow a normal districution curve, with a mean of 100, and the upper and lower limits of 141 and 69, respectively. We also know that you don't have to get 100% to get 141, and that a score of 69 does not equate to 0%!