The rear deck is curved for several reasons: It helps to reduce the loading angle by allowing the rear deck to nest over the tires when lowered.  This shortens the overall rear deck length while providing a very low load angle, which in-turn reduces the tail swing of the truck when driving. The curved deck allows low clearance equipment such as scissors lifts to be driven over the deck hinge point without bottoming out. Tracked equipment can be driven on without dangerous & destructive breakover.