As a kid, have you ever stuck your head out the window of a car while traveling at a high speed on the highway? Notice how your mouth fills up with air (cheeks/lips bulging outward due to the force of the air)? While I don't know for certain, I'm betting the physics are much the same as trying the same thing with your mouth stuffed with something (ie, a cloth bandanna, or whatever). The air would just then flow around the mouth opening. At higher force/pressures of a speeding bullet, the physics might well be different, but I'd assume it's similar to this mouth/air type of effect.