That tells you more about the inadequacy and bias of the IQ system in measuring and representing intelligence, than it tells you about African people.
If that were the case, what would explain the outliers -- high-IQ nations that are poor, and low-IQ nations that are rich?
Precisely that, they're outliers. There's fully grown human beings that weigh 60lbs and ones that weigh almost 1000, it's not normal it just exists because there's 6 billion of us on the planet.
There's hundreds of countries, so obviously not every single one will follow due to statistics.
Why do you believe there is inadequacy and bias in mental ability testing (the IQ system, as you put it)? Have you read this Citation Classic on the subject?:
http://www.amazon.com/Bias-Mental-Testing-Arthur-Jensen/dp/0...