Good article. As a scuba diver, it is curious to me how some people are really boyant while others sink! In scuba diving, you learn to adjust your buoyancy with your breathing and how much air is in your BCD and/or in your lungs.
What is the science behind this or is it just complex?
Just a guess, but I'd wager is has something to do with changing your volume while maintaining the same mass. Which would change your density and therefore buoyancy.
...or not?
I think so. I would also bet there is something too in the chemistry/makeup of the cells?
also, body fat percentage, bone density, muscle mass, et cetera.
True - all of that would weight in.