Haha.
Both the clavicular and sternal pectorals major function as a single force-producing muscle because of how they are attached, their innervations, and their functions. You cannot separate them short of breaking your neck (this would kill the innervations, rendering the muscle inusable). So, upper and lower chest = one muscle for all practical purposes. Now, when you put the bench on an incline position you put the shoulders in a stronger position and the chest in a weaker position. This means that the shoulders will do more work, and the chest less. If you put a bench on a decline, the involvement of the shoulders is minimized, and the chest is put in it's strongest position, thus the chest will do more work, and the shoulders less. Since the upper and lower chest work completely together regardless of bench positioning, the decline will work the upper chest more than the incline for the reasons above.