Individual instruction should increase test scores by about 2 standard deviations[1] but the linked results are only about 1 standard deviation above normal.
I think the explanation is that homeschool parents are not providing "tutorial instruction" to their children most of the time (that's the 2 sigma effect). I don't know of good statistics of actual parent behavior but from my homeschool readings it seems like parents often provide something like "reinforcement", "cues and explanations", and "student time on task". Those are 1 to 1.2 sigma effects.