see where the airflow departs from the roofline - that is where the air over the car transitions from smooth laminar flow into turbulent flow. at the rear, between the smoke and the car, picture air just swirling around with no cohesion.
note how the smoke has the shape of the upper half of an airplane wing. imagine the air under the car forming the lower half of the wing. the closer that transition point is to the front of the car, the less airfoil shape you have. conversely, the closer to the back it is, the more it looks like a wing.
ever notice how a big ship pushes a bow wave ahead of it? how does the water know the ship is coming? ditto spoilers. they create a larger pocket of turbulent air, which pushes the transition point forward and decreases lift.
the turbulent air has no cohesion, so does not provide any downforce by hitting the spoiler and shooting upwards. in fact, the air directly in front of the spoiler isn't even moving backwards at that point - it is just confused air.
this is why, if a car has a real, effective wing, that wing is high enough to grab the laminar airflow above the car. and realise that it is an upside-down wing, designed to use that clean air to produce negative lift that pushes down on the car.
this is also why the porsche gt3, etc., have both a spoiler (tailbase) and a wing - they serve two different purposes.