I had to check my recycle bin.
The idea here is that when seen from behind, the 'bumper' is only as thin as the liscense plate. It looks thicker from the side because there are actually aerodynamic extentions behind the wheels to clean up the airflow. ( Rather conveniently, this also makes the car look closer to "normal' , since it lessens the 'cone' look.
If you were to see the car from behind, there would actually be two floor sections to get the ideal angle from below. (There is a biplane sort of look to it. ) There could also be channels running lengthwise like those on diffusers.
( Note the mirrors, lack of any sort of grille block, no wheel covers, and all that sort of thing ... it's just a throw away image)