Firstly one must understand that there is no dictum in the field of medicine which pronounces that "A normal person's stool must sink every time to be considered ''Normal' ".
Stool is a waste product composed of remnants of the food that one eats + bacteria + gases.
Remnants of the food refers to the waste products of digestion, foods that can't or could not be digested(ex; excess of dietary fat).
Stool that has an excess of undigested fat will tend to float.
Stool which contains excessive gas will also tend to float.
If stool consistently floats every time for a prolonged period, then one should investigate for 'Steatorrhea' (where the stool is slimy,oily, sticky, foul smelling and difficult to flush).
'Occasionally-floating' stools have no clinical significance. Best wishes!
MB BS, MS, FRCS(Edinburgh)