This might sound a little fussy, but. After the fabric is shrunk, the ribs will be in tension against the weldment and the Hysol fillet. In the earlier models with the standard simple flat rib the tension would occasionally result in the rib warping. The stiffener was added to prevent the warping. Extending the stiffener to the weldment will add unnecessary weight for very little additional benefit. Ounces become pounds.