Both my daughters have scored in the 99th percentile too.
My experience with asking teachers about nonverbal has been the same as yours!!
Some vague statements like 'they are tested in their reasoning capabilities' to 'It's an IQ test'...
But on asking my daughters about the questions in the test, it sounds like puzzles, picture and numberical patterns etc seems to be the method.
The question is how do they know if the 5/6 year old just doesn't feel like sitting and answering a bunch of questions on the given day?
