Naked eyes see very little when it comes to impellers, Morty. There could be tiny cracks you couldn't see in the blades, for instance.
But what I'm guessing happened is that the old one wore out and got thin, so it wasn't up against the faceplate of the pump housing. You'd never "see" that, and could only gauge with a pair of vernier calipers.
Also, sometimes the faceplates get scored inside. Either sand them down flat again with wet fine paper, or just buy a new one.
Speaking of spending your money, did you buy a replacement impeller while you were at it, as well as a gasket or two (which you can also make from thin paper bags)?