The reading fluency measure is the one difference. That is a timed test of reading sentences and answering questions. Perfectionism or just thoughtfulness or being tired or any thing could throw it off. It also depends on baseline knowledge external to the test and not contained as a pure skill test. I'd guess enough of the questions take a small bit more thought from the highly gifted thinker, because yes/no can get to be quite ambiguous when you look at things in a bigger light.