My take: they've become bored as hell, conditioned over the years of idiot level race-to-the-bottom schooling to hate doing what they're assigned the way they're supposed to do it because they don't require as much repetition as other people apparently do in order to "get" things. To get any intellectual enjoyment, they had to find interesting stuff on their own.

I'd also suspect that the introvert part comes about mostly because of the various social stigmas against doing well in school; later in life, a lot of people that were silent outcasts in school really come out of their shells once they're around people that value skills other than throwing balls around.

In other words, my take is that there may be a causal relationship here, in that "giftedness" (whatever that really means) tends to force people towards a certain personality type in most high school environments.

I'd be very curious to see if these results continue to hold in cultures where there is less teaching to the bottom and more respect for academic talents.