the laser on the bottom is probably powerful enough to allow the sensors to still understand where the mouse is moving when its half a centimeter above the surface. Any more and the laser probably disperses too much. You won't find it happening with a ball mouse.
Is it optical? If so, it's common. Sometimes mine will move when it's sitting still. It's something to to with the laser that reads the hard surface. If you nudge it or pick it up and put it down, it will stop.