My guess is that the spoiler doesn't so much "create downforce" as it "reduces lift" at speed. As mentioned above, the profile of the car is somewhat like an airfoil and therefore it creates some lift, especially as velocity increases...that lift relies on a nice smooth, undisturbed (laminar) flow of air over the top of the vehicle...the spoiler...well... "spoils" that flow and lessens the tendency to create lift.

Bryan
(B.S., aerospace engineering)