It has something to do with harmonics. Every note comprises a fundamental frequency. The harmonics embrace a much wider frequency spectrum than the fundamental, which makes different instruments and different singers sound different when producing the same note.
This also explains why all speakers do not sound alike. They all do a different job (better or worse) reproducing the frequency harmonics. Even the same instrument will sound different depending where it is played. Different concert venues have their own unique accoustics that affect the sound. I am starting to talk in circles so I better stop there. Good Luck!