I've got a fairly high IQ, but what Steve said.
It doesn't measure how effectively a person could use it. It doesn't measure (and I'm commpletely with you on this) whether a person just stopped wanting to answer questions. I had disgraphia as a kid, so my first test was oral. I scored a bunch higher on that one than the one I took when I could write. And then, there was ADD, and getting to the point of "Aw, forget it. This is stupid". Some of the dumbest people I've known were doctors, who I'd guess, were good at memorizing stuff. Some of us just would rather figure things out than remember stuff.

And average IQ is ALWAYS 100. If we dumbed down, then possibly one would be able to score 100 if they could correctly spell "the", and count to 10. THAT is what can change, but 100 is always the average.