If you drill 1000 heads with the same boring bit, then all the holes should be the same size, and the same camshaft should fit in all of them.

The differences are where EXACTLY that whole was drilled in relation to the rest of the head. There are small differences there. That's why you can't always mix and match caps. The half-moon cut in the cap might end up being slightly offset from the half-moon cut in the head, and all of a sudden you don't have a perfectly round hole anymore.