Why do humans float when their specific gravity is greater than 1?

if the volume that is the human body (including its air-filled spaces) is greater than 1, the body will not float.  Fill the lungs with water (drown) and the body tends to sink.  It will arise after some time when decay gases cause density to decrease once again.

If you do a dead-man's float (a method of preserving energy if lost in open water and too tired to swim), you will see that the upper back is the part that rises to surface, because that is where the lowest density is (in the region of the lungs).

The human body cannot and will not float if its density as a body (mass per unit volume of space) is more than 1.

The specific gravity of the humans PLUS THE AIR IN THEIR LUNGS is less than one.  If they lose that air then they sink and drown.  It is like asking how can a ship made of steel float when steel does not float.

Politicians float more because they're full of hot air