If they all line up with the anti-billow arm on all the covers, I'm betting they wanted to give it a solid place to hit rather than the fiberglass.
Slide covers are held in via the internal spring in the roller. If the wind hits in the right way while driving, it can create enough force to overcome the spring and unroll the canvas. To solve this, they add the little arm which will hit something solid and stop it from unrolling (when putting the slides out, the whole mechanism moves away from the side, so the little arm doesn't hit anything and the canvas unrolls).
