IQ tests only measure how well one is at doing IQ tests. There is a correleation with school performance, but this doesn't imply that someone with a low IQ isn't intelligent.

I know that I have a very low IQ, I don't know exactly how low. But when I was tested in primary school to determine to what sort of secondary school I should go, the psychologists adviced that I was unsuitable for technical/mathematical stuff. This was much to the surprise of my teachers, I was the best of my class.

Since then I've done a few times similar tests, but each time the outcome was very bad compared to how well I was doing at school. The tests didn't measure IQ but were measuring what type of education is most suitable for you. But superficially the tests are very similar to IQ tests I've seen.

The only part of the test I performed well are the number series. I suspect that what's going on is that the test assume that to do well in certain subjects means that you must be able to quickly spot certain patterns quickly. I think that it is actually the other way around. If you can spot the patterns quickly you will probably be very good at certain subjects. However, it can be the case that your brain is wired differently and that you are good at seeing patterns at a more abstract level than is tested.

In that case you may have very bad mathematics ability according to the test, but still be very good at mathematics. Note that there are some tests in which Chimpanzees score better than Humans. If you train Chimps to find a way out of a labyrinth they'll point out the solution much faster than well trained humans can. The Chimp brain is apparently much better configured for this task.

Now suppose that the labyrinth test was used by psychologists to test for mathematical ability. Arguably there is some correlation between mathematical ability and finding your way out of a labyrinth, because you need some logical reasoning for this. Then according to this test Chimps would be mathematical geniusses.