I think other thing to note is that most of the schools have a net intake in Year 11, and few losses to apprenticeships / TAFE, which is the opposite of state schools.
So in some cases a kid who might be suspected of having a scholarship by third parties might be a genuine full fee paying student. Granted this is not always the case.
And parents do sacrifice a lot to get their kid to a school that meets their interests which will hopefully help them thrive.
If a kid is academically minded and interested in agriculture as a career, you'd be pushing them to James Ruse, if they are musically gifted it might be The Conservatory, even if another school supposedly has triangle scholarships.
And to be honest it's probably good for Australian rugby if a talented kid from Outer WoopWoop High ends up playing for a GPS school for two years in more competitive games. Less of a gain if they had been at Auggies or Oakhill.
And then you have the kids who play a different sport on Sunday, and who's school fees are paid by some corporate entity. A tough case - 90% are probably lost and are possibly preventing others playing reps, but just from the experience might you be able to keep some or tempt them back after a few years?