Sometimes it's a matter of vocal range (has this been said already?)- this is from personal experience. When recording, "Too Many Shadows"(album, None But One)instruments had begun to play, the tape was rolling, and I realized that in the key chosen, I would NOT be able to get the low note in the chorus-Bright sunshine gladdened all our WAYS- so whispered to son Peter,(bass), "Take that line with the low note!" He did, and I sang something higher, and I don't suppose anyone now actually knows that they're not hearing my "melody" right.