The programming talent myth

The uncanny valley!... Sorry... I bet you wouldn't see much bimodality if you got people to draw geometric forms or maybe architecture, but if you got people to draw faces and the human form, then it would appear, the reason being that the skill required before a face stops looking weird and wrong is quite high (though probably lower than many people think). The same could be true of programming... If you don't know how to get every part of your program just right it's behavior will be weird and wrong. (Though sometimes interesting and inspirational too)
That sharp divided between correct and broken would also help discourage people from learning, as you said, worsening the effect. That might even be the primary factor in the two hump thing, if it is a thing.