An IQ test is a relative measure of where you are relative to the population at the point of time where the test was normalized. That point of time is somewher

An IQ test is a relative measure of where you are relative to the population at the point of time where the test was normalized. That point of time is somewhere in the past. And when tests are renormalized, there is a conversion of "this score on the old test is that score on the new test". This allows for comparisons of IQ over time, across different versions of the same test. This is how the Flynn effect was first discovered. Tests usually have solid conversions between them, you can can compare across different tests as well. Such conversions allow more verification of the Flynn effect. If you go back far enough, you will find tests for children measuring mental age vs physical age, taking a ratio, and multiplying by 100. They fell on a distribution that was close enough to normal centered at 100 with a standard deviation of 15 or 16 that adult tests were developed to match them. Since we don't know all the factors behind the Flynn effect in the first place, we also don't know all the factors behind why it might be reversing now.