Examining some more actual photos (and the renderings), I am suspecting the interior sides of the bed taper slightly outwards vertically as well. It may be a few inches wider at the top than at the floor. I wonder if this explains some of the variance in bed width dimensions that have been reported.
Notice in this photo the slightly darker triangle back by the Tailgate. If the sides were vertical, that crease/seam between the side and the triangle would stay vertical. But its angled.