All the broadside examples I've seen have two, not seven, years; Ashton did make occasional errors of transcription, but I don't see any reason to think that he made one in this case. I suspect that the years stretched to seven when the song moved into oral currency; "seven years" is a nice, round, traditional span and would come automatically to mind.

The set Christie printed in 1881 (Traditional Ballad Airs, II, 100-101) was from tradition, with two years changed to seven and Phoebe's name dropped from the title, but otherwise hardly changed from the broadsides. Later broadsides sometimes dropped her name from the title while retaining it in the song (De Marsan of New York, for example, c.1860: see Bodleian, Harding B 18(114)) but on the whole it seems to have been oral currency that trimmed the song down and lost or sometimes changed the heroine's name.

There's no particular evidence that the broadsides are elaborated versions of some unrecorded folk-song (though they certainly follow the standard formulae for such subjects); more likely, on the whole, that the traditional sets are reductions of the broadside.