I'll take a stab as I really don't fully understand those little lip spoilers enough (enough to my satisfaction
)
This image shows CFD of a Mitsubishi Lancer IIRC. The right side doesn't have vortex generators (so ignore the left side). Notice how the blue area (lower velocity air) pretty much misses the trunk lid on a downward angle? By adding a lip there, flow is straightened out before leaving the vehicle and I would think would make flow interaction from swirling air below the car and above the car less of a problem...
If someone has a better understanding, please chime in
