Why are potentiometer shafts different? I.e. course/fine knurled and some solid-shaft, split-shaft? I understand knobs fit different shaft types but why are some bodies' routed so that only a split-shaft pot fits?
Whether the shaft is split or solid has nothing to do with the body routing. Some bodies are very thick and need long shafts, but I find that short shafts fit just about every guitar.
Split shafts are most common, and accommodate most push-on plastic knobs (like Gibson and Strats). Metal knobs with a set screw (like a Tele) work best with solid shaft pots.