Yeah, there's a lot of misconception about aerodynamics out there. Covering the front half of the bed reduces the turbulent "drag" zone and cuts drag behind the cab by 50%. Normal high pressure air fills the rear uncovered half of the bed and allows the air to seek a more laminar and uniform path over the tailgate without raising the high pressure zone up above the top edge of the bed like a full cover does. A tailgate net lowers the high pressure air zone below the top edge of the bed but increases the turbulent zone and extends it further back from the cab.
Combine a front 2/3 cover and remove the tailgate to get the best flow with the lowest drag (short of putting an angled cap over the rear down to the rear bumper).