The top 5 highest scoring countries, Hong Kong, Singapore, South Korea, Japan, and China all have a cram school culture that prepares them for the endless rounds of standardized tests they have to take to secure a position in society.
I would expect people trained from infancy to excel at standardized tests, to excel at standardized tests.
Many of the reasons I listed would explain why, in the US, the states of West Virginia (98.7) and New Hampshire (104.2), despite having very similar demographics have such different average IQs. One state is richer than the other.
It would also explain why Hawaii, which demographically is comprised of people (about 1/3rd) who share genetic lineage with people from high IQ nations, has a relatively low average IQ. Hawaii has a pretty bad public school system.
Until someone locks two children in an Edu-Pod from birth to adulthood and feeds them the same stream of information and nutrition and measures their IQ when the pod ejects them at the end of the test, we'll never know for sure, but my gut tells me that all of the people who think "genetics is destiny" are losers.