IHSA tries to keep it fair by counting students at private schools that operate without hard boundaries as being more special than students at public schools that endure the harsh realities that they can't recruit kids to come to their school. You are about to see a sh*#storm of comments about what a whiner I am and how harsh the IHSA rules are regarding this multiplier, but remember that every one of those nasty guys has attended, or currently sends their mutant spawn to said private schools. SF (sarcastic font)
The multiplier take the enrollment at an unboundaried school and multiplies that number by 1.65 to come up with an enrollment figure to match against public school actual figures.