Once the buyer has been convinced, his worldview is dependent upon a number of assumptions he's made by extrapolating limited information. He makes a number of those assumptions on the basis of trusting the seller. If that trust wavers, the assumptions crumble and the worldview collapses.
The buyer's trust in the seller is fragile because the buyer doesn't really know the seller. His image of the seller is based on an internal model that the seller has triggered by being similar on a few heuristic markers.
The buyer jumps to the conclusion that the seller is that good guy he's been waiting for, on the strength of a few matching signals.
But the more the seller says after that point to demonstrate defining characteristics of his knowledge, personality, or character, the more he risks varying from the buyer's internal model. Less is more, because by increasing the number of points of comparison, the more the seller risks spoiling the match.