Three ways words can change in the way you sing a folksong -

one is you heard it wrong

one is you remember it wrong

one is you deliberately change it.


And that last could be

because you think you have a better word or line that runs better,

or because the words don't make sense to you, because it's come to you in a garbled version, and you are trying to fix it,

or because the words as they stand don't mean what they are meant to mean, because language has changed.