ITunes video download music changed?

I downloaded some videos from iTunes. I got all of the episodes from the new MTV show teen wolf, and in almost every episode so far, not all of the music is the same as it was on tv or on MTV.com Is it because of copyright infringement or something? Please help me. I know I can't do anything to change it, but the amazing music on this show makes the show even better. Why is it when I downloaded the tv show episodes that iTunes or something has changed the original music that went with that episode in the first place? Any ideas why? Has anybody else noticed this when they downloaded tv shows or movies from iTunes?