It’s self-selecting statistics.
If you force homeschooler scores to be averaged with ESL, dysfunctional urban students and other underperforming students, the average would go down.
It’s not a slight on homeschooling, which is a good thing usually, but statistics is statistics. You do not want to give folks the impression that just by pulling a kid from one environment into another is alone responsible for improvement or degradation of scores.
That would be misleading.
Not all students take the SAT, only those that have a desire to go to college. The statistics for the public school students are not being lowered by “dysfunctional urban students” and other “underperforming students” because those students are unlikely to take the SATs in the first place.