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Christ, I get where you're coming from as on the surface it does sound like high speed air over the roof should be low pressure (and it is) while lazy air under the truck should be high pressure, creating lift.
TEiN hit on one of the points and that is stagnation pressure at the vertical air dam above the chin splitter. The stagnation pressure pushes downward on the splitter creating heavy downforce. The high pressure (but not quite stagnation) at the base of the windshield does a similar thing on the hood. The spoiler on the decklid creates a similar downforce on the tonneau. The combination of splitter, windshield, and spoiler downforce is designed to overcome what lift results in the center of the roof.
Additionally by having slab sides run near the ground and an extremely low air dam/splitter there is very little to zero air flow going under the car. Because nearly all the air is pushed over and around the vehicle the air from the undercarriage gets vacuumed out the wheel openings and the rear end to the low-pressure areas along the sides and tailgate of the truck.
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