Many geniuses tend to show such tendencies; Asperger's Syndrome runs in my family.For those individuals the higher the IQ, the more the condition manifests.
It would have saved my family a lot of time and trouble if we had known what was "wrong" with my brother and cousin and others. As far as I'm concerned, it's not a "pathological condition" but a subtle difference in brain chemistry.
As for mediocrity (or run-of-the-mill intelligence), perhaps that is in some ways requisite to happiness, or at least to contentment. That's not to say I think complacency is a good thing, though.
It may well be that their was nothing "wrong" with your brother at all, as you say.
Whatever roots psychology may have had in science has long since been left behind. It has become a religion.
I am sure that my brother has Asperger's syndrome, though he has never been diagnosed. The school district sent a team of representatives to our home to inform my parents that my brother had the highest IQ that they had ever seen in the school district and apologize for having bored him to death. Everyone thought that he was just weird and blamed it on my mother's parenting. A psychologist once warned him to never marry, telling him that he was not capable of bonding with another person. My brother is the most rational man that you would ever encounter, completely devoid emotional thinking. The only emotion that he exhibits seems to be anger.