This is done on purpose, because monsters appear while you explore the dancing room and the music gets more twisted before transitioning to "Shadowlord's Castle" (battle version). The transition within "Dance of the Evanescent" itself is actually pretty short in-game, and it's probably dynamic music, so I guess they've tried to make the track longer specifically for the soundtrack, so you can hear how it gradually collapses.
https://youtu.be/VmG8hz9RgvQ?list=PL...RtJfBUk&t=1023