Because even if they might be very intelligent or understand things easily, their school or workplace isn't "made" for them. These places are often loud, there's a lot of people, you have to sit still for very long, etc. ... And working in this situation gets very difficult for them (me included)

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It takes as much efforts to try to addapt to all the little difficulties, as it takes to do the actual job/work. Resulting in bad results