Distances between Upper & Lower Moldings

[My Alter Ego]

Recently bought a 71 Cheyenne LWB and I swear that the upper molding on the bed tapers downwards as it gets closer to the tailgate. I have always been under the impression that the distance between the upper and lower moldings was the same throughout the length of the truck bed. At the front of the bed, near the cab, the distance between the two moldings is 13" and at the rear of the bed, near the tailgate, it measures 11 3/4". Is this correct? Any enlightenment would be greatly appreciated, thanks in advance.