In the late 80's, while leading a folk guitar workshop, Martin Carthy was asked about the similarity of the tunes. He replied that when Fairport encountered the words to Matty Groves, it had no tune or final verse. Fairport adopted the Shady Groves tune well known in Appalachia, tidied up a few lyrics, and added (or rewrote, I forget) the final verse to come up with the version we all know.