IQ measures how people, usually kids, respond in a test taking situation at a given time. This doesn't necessarily accurately measure how those same people will respond to other situations. A snapshot of behavior is only so good at predicting aptitude/outcome. Most obviously, people who are currently hungry or scared for some reason don't focus as well and perform poorly. People with low expectations of themselves also perform poorly. You'll get more stereotypical performance on tests if you remind people of those stereotypes before the test. People who you would stereotypically expect to perform worse tend to do so. So, IQ tests actually measure something, but it's the combination of a lot of factors...