Well, I just disagree. I've been doing this professionally for over 10 years and some people are bad to okay at it, and others are good to excellent. If there isn't some deep-seated reason why people fall into one group or the other, if it's just practice or whatever, fine. I don't know what the explanation is. And I don't assume it's impossible to move from one group to the other, far from it. But I would say the bi-modal thing seems roughly correct.