What do you think a variation IS? SMW uses the same theme for continuity, and source material. The entire game has an incredibly unified sound track because of all of this. And it wasn't because of a time issue. As a composer I can assure you that making the same theme have 20 different aesthetics and moods to it is just as hard, if not more difficult in a lot ways, that creating a completely unique theme for each area. That same Theme has to be able to be manipulated, played with and shoved in to every kind of mood that you want it in. In no way is it getting out of anything by designing a soundtrack this way. It's a compositional choice, and definitely not a lazy or rushed one.