You got the facts right but on some of our less aerodynamic designed machines, the air is turbulant and at lower pressure than further behind the rear of the car. It isn't a vacuum, just lower pressure and the mist or whatever comes from beneath the machine is lifted up and into that lower pressure area. That's when the heavy stuff falls out and onto the coach work.
Boy you didn't know I knew so much did you?

Happy Trails Beep Beep from fallout Houston.
Donald