It's like there's always one tiny hair or little speck of dust just lying in wait while you diligently perform cleaning and blowing, checkingetc. Static can hold such things inside the case ready to fall straight on to the dial the minute you screw the case back down.
The old boys, like Donald de Carle in "Practical Watch Repairing" - a highly recommended book if you haven't got it already - I know you have Karbon - advocates holding the case in one hand and taking a small flame and passing it round the inside of the case to eliminate this and it does work. Diligence is required obviously - watch that plastic crytsal gasket, any rubber components, etc. It should be performed fast but smoothly, never hovering in one place for a more than a moment. It is the last action performed before re-casing the dial and movement.