It's worse, because every test is obviously bounded, and it's absurd to not expect some noise to be there.

Join those two, and the test only becomes reasonable near the middle. But the middle is exactly where the pick of questions makes the most difference.

All said, this means that IQ is kinda useful for sociological studies with large samples. But if you use it you are adding error, it's not reasonable to expect that error not to correlate with whatever you are looking at (since nobody understands it well), and it's not reasonable to expect the results to be stable. And it's really useless to make decisions based on small sample sizes.