Well, that’s partly because IQ is not necessarily an actual measure of intelligence. There’s a lot of debate about this (both in terms of problems of conceptualization and measurement validity). But we do know that IQ tests are culturally biased in many ways.
For example: When we moved to the US (from Bolivia), my younger brother (Sam) and I were given IQ tests to determine whether we should get into the gifted program. I was about 10 years old, and spoke Spanish and English. Sam was about 6 years old, and didn’t speak English (only Spanish).
I struggled with the IQ test, but passed. I remember one particular tricky problem that asked me to look at an object and match it with another object. In this case, it was a butterfly. My two options were a nail and a net. Now, I had never seen anyone catch butterflies with a net (it was a “foreign” concept to me). I knew the nail looked “off,” but at least I had seen butterflies pinned to boards at museums (and I had an uncle that had a lovely pinned butterfly collection on his wall back in Bolivia). So I chose the nail. I associated it with the butterfly. That choice marked me as “not bright.”
My brother had a tougher time with his IQ test, since it was administered by a monolingual English speaker and he was a monolingual Spanish speaker and he was only 6 years old. He didn’t get into the gifted program. Nevertheless, he went on to become an engineer and could hold his own in political theory discussions with my graduate school friends when he was a freshman undergraduate. Oh, and did I mention that he could estimate a calculus function in his head (stuff like that was a great help during my graduate methods courses!)?
But the IQ test weeded Sam out of the gifted program because he was a confused, recent immigrant non-English speaker asked to do a lot of random tasks by a stranger who spoke to him only in English.
C’est la vie, I guess.