Original soundtracks

I have been reviewing my music library in iTunes on my MacBook Pro. I have been selecting artist and then looking at the albums I have for each. Artist are listed alphabetically, and I got through the "N's". When I started to review the "O's" there was a listing "original soundtracks", not an artist name. There were 62 albums in this listing. As I reviewed them, many were complete albums and some were a few songs from the album. The rest of the album was listed under the artist's name. Any ideas on why iTunes would file an album under original soundtracks when the album is not a soundtrack?