Partial quote of your post:
I've seen swimmers exhale almost all air in their lungs and remain at the surface (lucky!), while on the other extreme, some will head to the bottom after just a few seconds of exhaling (sinkers!). The majority seem to be within 60%-40% lung capacity before falling to the bottom of the pool.
Just curious, but why does that happen? My husband could not believe that I could not sink to the bottom when exhaling air. He pushed me down and placed his foot on my back. When he released it, I popped up like a cork. Could it be that I still had some air trapped in the lungs somewhere?