Very true. One of the biggest problems with the whole IQ thingy, is that you are trying to assign a number to this basically abstract concept called "intelligence". It cant be realiably or realistically quantified, no matter how good the test.
You can make your plastic parts all day, and they arent going to change just because you got into a fight with your wife the night before, or you drank too much, or you are excited about the weekend.
With an "IQ test", lets say i score a 130. The next time i take the test, maybe i score a 110. But what the score doesnt tell you is that i was up all night right before the test. So is that error in the test? No, that variance in score is attributed to being a human and having roughly 8,000,000 affecting your performance at any given time. I didnt "regress toward the mean" because there was error. I !@#$ed around all night, got no sleep, and then did piss poor on the test. The test was fine. it was ME that wasnt.
You cant reasonably expect to assign a number to something like "intelligence" when there are a ton of factors and forces that effect it.