[pedant]
Actually, at least in the case where you see lyrics reproduced on an “official” product (as opposed to a fan’s transcription on the internet, differences in lyrics between what is sung and what is written are usually attributable to the lyrics that were submitted when the copyright was registered for the song. The copyright for the musical work (which can include accompanying lyrics) may have been registered before the artist decided to change the lyrics while recording them, or the artist may have chosen to register slightly different lyrics for purposes of those different lyrics being used later in songbooks, etc.
(A good example of the former is “All Dead, All Dead” by Queen, which contains an introductory verse that was later dropped from the record, while an example of the latter is the song “Ignoreland” by R.E.M – on the record, Michael Stipe clearly sings “my, CBS TV tells a million lies” – since that would have been clearly libelous if reprinted over and over again in a songbook, the copyrighted version that appears in liner notes and songbooks states that the lyrics are “my skin, my ass, TV tells a million lies.” )
[/pedant]
M