Folk tunes have typically been passed aurally from player to player over a number of years. Transcriptions are only interpretations of the tunes; many players embellish the tunes as they play.
Myself and a couple of friends when leading a tune session sometimes meet people who have just learnt the tune from a score and will suggest we are playing the wrong notes. Our usual response is, this is how we like to play it, we like the way it sounds. Music theory is OK as long as it doesn't get in the way of actually making live music.