This could also be a selection bias. Not that many kids take the PSAT in 10th grade- those who do are likely going to apply to very competitive colleges. These are kids who want to get experience taking the PSAT a year early. These are also kids who would probably do well on the AP exams. It could also be that many kids who take the PSAT in 10th grade come from above-average income families, and those may be kids who will take alot of AP classes and exams anyway.
In our local, highly competitive public school, alot of kids start taking the PSAT in 9th grade.
