You'd be surprised how little amounts of dirt/dust effect the smoothness drastically.
Slightly off topic- Ceramic detents can become especially stiff since the ceramic ball is harder than the steel and can cause swarf to pile up on the detent path. Don't believe me? Take a ceramic detent knife and open and close it (don't lock it) repeatedly for a few minutes and you'll notice the smoothness slowly disappear. The toothbrush trick will instantly restore smoothness.