I'm um, rather new here *cough*, but I was recently reading about this topic and I just glanced over whats been said and I don't think this has been stated yet, but...
Except for the normal variations in intelligence between everyday individuals each race proves to be basically the same in terms of intelligence.
Ignorance among those of the african american persuasion arrives due to disparities and the lack of good education in the areas where they are concentrated, urban detroit for example. In detroit how much time do you think is burned by disruptions or fighting between students because of the propegation of the problems in their area. I hate to forward the stereotype but in a lot of situations it can be very difficult for a black person to 'get out' of the ghetto or whatever you may choose to call it.
What you are comparing is clearly not intelligence but education. Consider how you said Orientals rank higher than others, well, the oriental school systems (China/Japan) have classes that run one or two hours longer than north american ones, not to mention it is ingrained in their culture and familial structure that they are supposed to study hard and work.