Well, on the soundtrack, the overture is titled "Ilia's Theme," not "Overture." So I've always kind of figured that Goldsmith used the theme as the overture because he hadn't been able to make a full statement of it elsewhere in the film, just variations and snatches. Normally an overture -- to an opera or a film score -- is a medley of themes from the main score (not unlike the structure of movie end title music). The West Side Story
film overture is a case in point. But this is just a single cue, leading me to wonder if it wasn't originally meant as the overture.