It can be a result of a "light" load, though if the pistol operates properly its not too light.
It can also be the result of a "normal" load and "springy" brass. It's the same visual result as incomplete obduration (sealing to the chamber wall), but it happens after the bullet is on its way (or even gone), as the pressure drops, the brass springs back away from the chamber wall (so it can be extracted) and if this happens while there is still some residual pressure in the bore, some gas will "leak" down the side of the case leaving the smoky discoloring.
You'll find it more prevalent with some powders, than others, as well.