The reason how we distinguish the theme of soprano sound from other instrument sound is easy. As when we play the piano and listen to the sound, the highest sound is soprano. So when we play the piano with the right hand, the finger for the end of right is soprano. So it is supposed to fit to make soprano sound distinguished from other part of sound. Also the highest voice is soprano.
The highest sound can be suddenly low when we consider this high sound as not the soprano note but the highest sound as the flow.