Matt, try thinking of it this way (and please, somebody smack me upside the head if I'm getting this wrong).
Every action has an equal and opposite reaction. So each time the car acts upon the airflow to change it's direction, a force is exerted on the car as well as the air. So when the car deflects air upwards, the air exerts a downward force on the car, and vice versa. And each action/reaction takes energy. So even if the net force is zero, if the air has to go up and back down, it takes energy.
When working with air flow at subsonic speeds (I think we can all agree that this is subsonic) it is generally considered incompressible. This is not to say that air is incompressible- it's just that the effects of compression in these situations are small enough they're worth ignoring.
So anyway, when the air passes under the car, the belly pan deflects it downward (note energy loss) and then flattens it out (note energy loss). As John was saying, this reduces the cross sectional area of the "tunnel" underneath the car. Since the forces aren't enough to compress the air a noticeable amount, the only option the air has is to speed up.
Think of it as the proverbial water in a pipe. If the pipe becomes smaller, the water speeds up. Same diff.
And as far as the air behind the belly pan, it becomes a tremendously disorganized flow that my feeble mind can only guess at.
Way not to speak to the question, huh?
And BTW, thanks for that glamorous pic of my truck. To be honest, I never really looked at it that way before.