the only thing I can conclude.. is I know the louvers build lift at high speeds... the airflow flowing UP the rear hatch from the low pressure, and everyday observations on a dry clear day, the louvers do lift at high speeds.
The snow observation was just for kicks, however due to the loose, coarse surface from the snow and ice, generated tons of drag and altered the aerodynamics of the car rather dramatically.
All i can say is louvers do lift at high speeds, which means pressure drops due to the louver design.