There is more going on with the results than pure personal intelligence.
Look at the racial result the bottom countries are all Black, and undeveloped while the top countries are European and Asian and highly developed.
I think it could be a function of the education system, or the access to educational information, maybe just access to the internet .
I was thinking that was a strong possibility also.
For instance, many questions on an IQ test are number progressions. If you have very little need in your society for using advanced math concepts, you won't do well no matter how powerful your intellect might be.
Remember the movie "The Gods Must Be Crazy". The starting location of the story was a very small group living on the savannah someplace in Africa. A empty coke bottle was dropped into their midst by a fly over airplane. It was the hardest item they had ever encountered and they found many uses for it. How well would those people do on any IQ test?
I thought maybe all this might be taken into consideration since the authors of the survey were psychologists, and might have developed methods to balance the results, but who knows.
A factor that might have an strong impact is the early diet of the folks in the country, and it would correlate with the poverty bias Tom observed.
The amount of protein in the first few years of a child's life has a MAJOR impact on brain development and IQ (my tin hat inner self wonders if the undeserved bad rap meat is given is part of a plot...I suspect it is). So, in poor countries or underdeveloped areas, the diet would likely contain lower amounts of protein and what was available would be allocated more to the adults.