When the song collectors were going round at the start of the last century, they'd find that singers would break all the rules. They'd change the metre, they'd sing notes that weren't in the appropriate key, they'd vary the tune from stanza to stanza, they'd vary the length of the stanzas.

So a lot of the time they'd treat these as mistakes to be corrected, and see themselves as "restoring" the song to the way it was supposed to be. (And maybe clean up the verses while they were about it.)

And that was even without having computers. I suppose now they could rectify the field recordings so that the singers could hear what it was they were supposed to be doing. And then do it again, but this time do it right...