I think the point that was being made was about gifted underachievment. Sometimes kids with very high IQs do worse in school than kids with IQs that are high but not gifted or HG high.

This idea also applies to standardized testing. Gifted people may overthink the answer choices and pick the "wrong" one (I'm thinking specifically of verbal questions with no objectively right answer.). I suspect that the same thing happens in the classroom. Gifted students may not accept the teacher's explanation, especially in math class where the approach may just be memorize and regurgitate.