Something to do with “regression to the mean” perhaps, like when black boys from relatively wealthy families still get lower SAT scores and have higher crime rates than their white counterparts, or even whites from less-wealthy families. I read some interesting results from a study including Prince George Co regarding that.
It also has a lot to do with what classes were taken in HS. Check out this quote Ogbu wrote in his recent book, Black American Students in an Affluent Suburb: A Study of Academic Disengagement, "that black students in the affluent homes of doctors and lawyers are looking at rappers in ghettos as their role models."