I'm not sure that's necessarily the case, Anglo. Gordon Hall is an obvious example of a recent 'traditional' singer who collated texts to come up with his 'best' version, and I'm sure Jim Carroll mentioned on one thread here that Walter Pardon did too. Other accounts have spoken of traditional singers using broadsides or books to fill out their lyrics. Maybe most singers throughout history simply sang what they'd themselves heard, as accurately as memory would allow, but I'd bet that a few have always tried deliberately to improve either text or tune, or both.

On a more speculative level, I've always been fascinated by the conversion of the maidservant accomplice in British Isles versions of 'Young Hunting' to a manservant in Frank Proffitt's 'Love Henery' a change which (as the wonderful and sadly late Tom Gibney used to mention in his introduction to the ballad) allows the murderess the opportunity for a seduction attempt. Was this change a matter of memory lapse, or a deliberate attempt - presumably before the song reached Proffitt - to spice up the story with a bit of sex?