It comes down to the yarn, and how much of it goes in.
An ordinary cotton towel is made from ordinary towel yarn. Shiawase is spun at 100 count — about a fifth the thickness — and Shiho at 140 count, roughly a seventh, which is the limit of what a towel can be woven from. Finer yarn means more threads in the same space: Shiho is woven at about six times the usual density.
That changes three things you can feel. It takes up water faster — laid on water, Shiawase and Super Marshmallow begin to sink in under a second, where an ordinary towel sits on the surface. It sheds far less: Beauty Skincare leaves under a tenth as much lint in the wash. And it keeps its softness longer, because long, well-made pile holds its shape through washing instead of matting down.
All of these are UCHINO's own measurements against a standard cotton towel.