@CheerfulMuddler here's how it was explained to me the first time we went through it:
All DC take the 11+ test - local and out of catchment (OOC). Bucks knows how many grammar school places they have - let's say 1000 places for ease - so set the 121 qualifying score at the point where the top 1000 DC are at 121 and above.
Let's say 500 of these DC are OOC, they have not only taken a 121+ 'spot' (placed in the top 1000 places) but they have also artificially raised where the qualifying score is hence local DC are also pushed below the artificially high qualifying score.
I mention London as that's the biggest OOC cohort but you can see DC across the country taking the test. From this thread, you can see that the London grammars are highly competitive and very challenging so these super bright DC taking the Bucks test really do scupper the qualifying score for local DC.
The reason why Bucks allows anyone to take the 11+ is a legal one - there was a case years ago where a person sued either a borough or the Dept of Education and set a precedent that DC should be allowed to take any selective test. I don't know the ins and outs of it. Hence my DC could take the Trafford test if we wanted to.
It's a really rubbish system ☹️