Hi David, yup Burr's got it nailed above - because that little segment that Burr circled above is not quite fully horizontal, you end up with a kind of irregularity at the center area of your revolve. It's very small in size so you have to zoom in very closely to see it:





It's not quite a regular hole, it kind of collapses on the other side of it. The other collapsed side probably makes it into a self-intersecting object, and there can be various problems with doing intersection calculations on an object that has self-intersecting pieces in it.

- Michael