"what happens then is the frailty of the human cerebelum and Mondegreens."

That's a rather negative and degenerative view. Of course, forgetfulness and misunderstandings play their part, but often a singer would make deliberate changes, adding or subtracting to make the story more understandable or better to sing.

There is also evidence that many of them did not have a fixed idea of either text or tune, and would "make it up as they went along", not in the dismissive modern sense but more in the way of a good storyteller or comedian, who will create their tale around a fixed structure of events, but never the same way twice.