Remember, air flow changes around the truck at different speeds. The Mythbuster deal was at highway speeds. The top of the truck has a "Camber" effect like an airplane wing from the curvature of the cab. By no means is it aerodynamic, however, you are creating lift from the cab. the faster you go, the more lift you are creating. That said, as speeds progress, the lift vector will tilt more rearward and create more induced drag, (drag from the lift vector tilting aft) to go along with the parasitic drag, (shoving a brick through the air). By removing the tailgate you are increasing the distance the air has to travel over the truck, thereby increasing the induced drag. Having a bed cover or wing on it reduces the induced drag and actually allows the airflow to separate from the truck sooner, creating less lift, effectively giving you more downforce.
I understand its far more complicated than that, but thats the readers digest version. IMHO.