What you're saying may be true if the radiator passed air through the hood at the same rate as if it wasn't there at all. But the radiator does present a small pressure drop across it, thus bringing a closer unity between the opening area, and the radiator area. Secondly, air is going to follow a path of least resistance. If there was no lower wall or fence, air would see the radiator more of an impedance than the open area underneath it, and thus the much of the air that would normally be forced through the radiator would instead simply flow under the car. In my particular example, once get up to about 25 MPH, I no longer need the fans. Even with some very tight fan shrouds, and the fans off, enough air still passes through the radiator to cool the engine above this speed.