I am far from being a sailor, but my guess would be air trapped in the hull, and possibly built in buoyancy. I once read a book 'bout a group of guys to set out for New Zealand from OZ in a triple cat sailboat. Somewhere out there they rolled it over, ended up living in the upside down boat for something like two months before floating ashore back on OZ. They even cut holes in the "bottom" of the boat so that they could to into the hull and then back out on "deck"...