I'm surprised you've chosen to point to IQ and the Wealth of Nations. It is not exactly the paragon of scholarship. As has been mentioned before, for the majority of those nations, no study was even available, they extrapolated from neighboring nations. As the result we can posit that the lowest scoring nations (at least those distant from one another) actually had studies, of some sort done (let's not question that validity of said studies for the moment, even though they should hardly count as IQ tests).

Let's examine the neighbors of the five lowest scoring nations:

Zimbabwe, The Democratic Republic of the Congo, Sierra Leone, Ethiopia, Sierra Leone, Equatorial Guinea.

Each of these nations are fairly distant from one another (ie: at least two hops on the world graph). Yet if you flood fill at a depth of two from each of them, you discover something pretty interesting: nearly all of Africa is filled! These countries were given a score of about 64, a bit more than two SD's below the world average. If an averaging of the scores of nearby nations took place, and it must have by the author's own admission, then the mere presence of these five low scores depresses the scores of the entirety of Africa, and this negative influence spreads all the way to the middle east. There's your consistency.