Oh, the story goes back a long way, and turns up all over Northern Europe; Lajos Vargyas thought that it came ultimately from Central Asia more than two thousand years ago, but his (iconographic) evidence seems to have been slender, to say the least (ref. A.L. Lloyd, Folk Song in England, 1967).  However that may be, the extant English language sets of the song that we have are, as I said, relatively recent, with a first appearance in print in (if I remember correctly) the late 18th century.  There isn't much point in trying to analyse this kind of song according to a presumed internal logic, because most of the time there just isn't any; for myself, I'm happy enough with the supernatural explanation of some of the older sets, but I doubt if the majority of traditional singers in the last couple of hundred years saw the song as anything other than an entertaining melodrama.

Malcolm