voice changers typically apply a filter of some sort, and these filters do alter the sound quality of the original. You could take a piano playing a middle C note, and use a filter to "create" all the other notes - it would not sound like someone actually playing a piano, although the actual pitch of the notes would be spot on. kind of like how auto-tune works. If you make a small pitch change you may not notice it, but make large enough changes, and you end up with a synthesized voice that is easily apparent to most listeners.