Complex sentence: Sometimes all of the tracks from the movie are included in the album, but more often this is not the case; however there are rare cases of songs in the trailers that do not appear in the movie but occur on the soundtrack album.
Split into simpler sentences: Sometimes all of the tracks from the movie are included in the album, but more often this is not the case. There are sometimes instances of songs in the trailers that do not appear in the movie but occur on the soundtrack album.