Ah you mean the performers, not the writers... (be clear next time), maybe they did do the other song in the movie (wouldn't be the first time this happens. or when another artist than the original one performs a song for the movie)?
Though it can indeed be a goofup (wouldn't be the first time either in a creditlist). If it is a goofup, you will probably find this listed on several (fan)sites on the net which go about this movie. Search google.
extremly quick one click google search:
http://www.imdb.com/title/tt0430308/goofs
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