It has to do with an aptitude called Pitch Discrimination and, unfortunately, you're either born with it/develop it or you don't. I remember my music teacher trying to show me the difference between how I was playing and how she wanted me to play, and I just smiled and nodded despite not getting it at all. XD"Can't you hear it?"
Hate people like that.
Of course, it depends on the situation, I guess musicians are sort of expected to have some sort of distinction?