So, when I was involved in this stuff, this was my best understanding:
1. Tests are tools. By themselves, they tell you nothing of real value. They are useful in the hands of a qualified professional.
2. Tests are game-able. They try to keep questions and tests under wraps because studying for the test or memorizing answers invalidates the test results.
3. Bright kids are horrendously hard to assess, especially if they are 2xE. They have trouble giving the canned "correct" answer.
This is something my son calls "guess the teacher's password." In other words "what inane thing do I need to parrot word for word to get the teacher off my back."
Example from email list discussion:
"Who discovered America?"
Teacher's password: Columbus.
Gifted kid's reply:
"I'm not sure. Columbus was trying to find India and thought he had, which is why Native Americans were called Indians. But it wasn't called America until years later when Amerigo Vespucci traveled to the continents now named after him...(blathers on about more history)...um, what was the question?"
4. Only certain tests can be used to assess IQs above 145. They were having trouble developing and validating an updated test for the higher IQs.
5. You basically can't be accurately assessed by someone with a lower IQ. Smart kids are routinely given hell by adults of more average intelligence who interpret their behavior as bad behavior and can't accommodate them.