People who sing pop songs work with a copyrighted text and melody. They may, of course, deviate from it. Some common reasons are a) failure to learn words or music properly at first, b) forgetting words or music with time, or c) desire to 'improve' or expand text or melody. People who sing songs traditionally are subject to the same agents of change.

When we hear an 8-verse song and the 6th verse of it has only 2 lines while all the others have 4, we suspect 'a' or 'b' above. When we hear a someone sing a new verse or new line in an old song, possibly with reference to a recent event, we suspect 'c'.

The traditional singers I know come in 3 kinds: 1) those who sing every song exactly as they heard it and would never change a word, 2) those who alter songs rarely (perhaps by localising a place name or varying the melody slighty), or 3) those who sometimes treat a song as an architect treats a beautiful old barn that his client would like to inhabit.

The great Ozark singer, Almeda Riddle, sang 'sWord' in 'Bingen on the Rhine' though, we can well suspect that she knew the W in sword is silent. So many place names have been localised in so many folk songs that there's no need to give an example. The great Paddy Tunney is said by many who knew him well to have made up the much-loved verse in 'The Green Fields of Canada' that includes the phrase, 'the fiddlers who flaked out the old mountain reels'. Jean Ritchie's father was the schoolteacher in Viper, Kentucky. He had many ballads with much fuller texts than collectors typically found in the Appalachians. Why would the schoolteacher not have the best texts? He could read and had access to books.   

Singers take pride in their songs. Some because they are 'exactly the way a forebearer sang it'; some because they are 'the original', some because 'they are from here'; and some because they are 'different' or 'best'.