Alternator charging Question

That is what I think it has to be. When I told him I have two cars that are charging just fine, but when I swapped this other alternator into either one it over charges, why could a rebuilder not tell me what it is right away, I figured it would be a snap for him.
It has to be an internal problem, so it should be easy to fix it would think.
Thnaks again 67 Dart.
AL