not uncommon.
Wow. I am a regular lap swimmer and water brat who learned to float as a baby. I cannot remain in a backfloat unless I move hands and feet slightly — my legs sink. If I fill my lungs with air AND do a Dizzy Gillespie with my cheeks, filling them up with air, and extend my neck and hang motionless in the water, I will end up floating vertically. It’s wild! But does that mean I tend more toward negative buoyancy? The only people I’ve seen who could maintain a backfloat without some motion of legs or hands, were ROTUND. I’m not exactly skinny.