Graphics updates related to new tech. If you're asking why Nova looks different in say the heroes cinematic, than in wings of liberty sc2, than in NovaDLC; Different artists have made those different models, so each one is different and representative of their work style and inclinations.
Art direction was different for each one of those too. While Heroes cinematic Nova looks spunky and youthful, Nova for DLC was directed to look more... rugged I guess.
The only Nova I've personally done in the newest campaign version, which if you pair it up with WoL Nova, it is quite different again (imo better :] )