Can someone explain this? this tends to mean that you get a book of business which has loads of little concentrations in them - you’ve got all the mixed-race d

Can someone explain this? this tends to mean that you get a book of business which has loads of little concentrations in them - you’ve got all the mixed-race dentists in Yorkshire, or something. And this, in turn, means that when the world changes, your risks tend to be very correlated and you lose years' worth of profit in one lump. I would guess that many small concentrations would be less correlated with each other, not more, than a few large ones. Or is he comparing it to having fewer 'concentrations' (which I understand to be market niches you dominate) at all; that is, having a few customers in this niche, a few in another, rather than everybody in a particular niche.