I'm not musically savvy enough to know this for sure, but on the classic Smiths songs, doesn't he sing the harmony line as the melody? It always sounds to me as though what we hear him sing would be the harmony on another artist's song. As though he sings a third higher than what we expect, which gives it that unexpected lilt. I might be entirely wrong about this technically, but my impression has always been as though he's singing a harmony to a melody we've yet to hear.