As small as lawn tractors are, it's tough to make the styling distinctive. As for the frames looking similar; once again all the manufactures use off the shelf components from B & S, Kohler, Tuff-Torq, etc and the mounting bolt requirements of those components will dictate very similar appearances of frames, etc.
As for small components like gas caps, etc; once again these companies source their components from whoever specializes in them and/or offers the best price. The VW Rabbit and Porsche 924 shared the same gas cap; but I doubt very much that they shared major components and assemblies.