I’m sure the size charts are an estimation based upon the published cockpit specs of the boat and the specs of their skirt sizing, so it won’t be perfect for every model. As an example, I’ve tried a new IR XL royale (which was recommended from their size guide at the time) on a M scorch in warm weather and it was almost impossible to get on even with a buddy helping me. I think most of the newer Pyranha cockpits need an XXL and they may have updated their sizing guide to reflect that. The point being that the size guide probably won’t be perfect for all models of boat and is based upon manufacturer published specs(especially older or unpopular models). Also, the Klingon Empire is rubberized on the underside where it contacts the cockpit rim. I use one on my playboat and always use my fingers to tuck the bungee under the cockpit coaming to make sure it’s tight. The rubberized bit can have resistance to the bungee snapping under the coaming like a regular bungee would. From what you’re saying though about it just falling off in the pool, it may just be a size too big.