Good Question. I lived with the same issue for years, but after reinstalling my ignition switch last night (after refinishing my dash) I believe I have the answer.
My ignition switch has a chrome bezel that screws on from the outside of the dash and a nut on the backside (behind the dash). The bezel will only go on so far before it bottoms out on the switch. The nut on the back side can be adjusted/tightened to give you the desired fit.
Hope this helps!