The weight of the wing is negligible for wing loading calculations. I would guess that if a lighter wing has a higher rating it would be something else in the design. For example, unsheathed lines. As for Ozone, where do you see that the lighter wings are more difficult? On their website, it looks like Roadster 2 and Spyder, which are essentially the same wing except for the lighter fabric, are the same difficulty level. The same is true with the Speedster 2 and Sirocco 2.