It is due to the fully boxed frame in the rear. The older Ford SD's had a C-channel frame from the bed back which one of its benefits over a fully boxed is a lower bed height along with easier upfitting. The downside to a C-channel frame is that it flexes more. This however should not be confused for being weaker as many believe when they see those GM and Ram propaganda video's of the frame flexing in the SD trucks. A fully boxed frame is more rigid than a C-channel frame, but that doesn't necessarily mean it is stronger when comparing two frames. I personally would rather have a C-channel frame with a lower bed height than a fully boxed frame with a higher bed height.