The use of averages could be misleading here. It could be that the US has the same pool of skilled developers as China/Russia but also has more unskilled developers, making the average lower.
(1) Your guess is as good as mine.
(2) Any relationship between IQ and developer skill is left as an exercise for the reader.
The real interesting comparison lies elsewhere. Also we have to take into account that we can not use the same tests on intelligence on both populations.
I actually think that intelligence test questions have highly correlated difficulty when given to people from different cultures (even more different than US/China). I.e. there's broad agreement in terms of what questions are hard/easy.