Why is the Imperial March from the Empire Strikes Back different from the soundtrack version?

When John Williams wrote the music for Star Wars he gave different characters different "theme" songs.  Princess Leia had her own, Yoda had his own theme, Luke, and others.  Darth Vader's theme was the Imperial March.  The soundtrack takes all of these different themes and mixes them together in different ways to fit what's going on in the film.