The scene from the promo was most likely raw footage, taken directly from the boom mike on set. The line from the actual episode was then redubbed using a technique called ADR (Automated dialogue replacement) where words that are slurred or pronounced with a colloquial accent can be corrected later.
According to the show's 'dialogue rerecording mixer', Chris Haire, some episodes had as much as 50% of all dialogue being redubbed after the fact (and not just the lines spoken by people in latex makeup either).
Still, technical problems are not the only cause of dialogue replacement. The producers place a lot of emphasis on the clarity of speech, and that means not only rising above extraneous noises, but also steering through strong accents, masks and false teeth. 'It's a very dialogue-orientated show in addition to the sound effects.' says Haire. 'A typical Star Trek episode averages about 150 lines of ADR, and a heavy show will be in the 300s. 300 lines is about half--or maybe more--of the total. That, however, is down to the fact that the producers want every word to be pristine; clean and perfectly spoken. If a word is slurred or its pronunciation is too colloquial, then the actor will be brought in to redo the line.
Richard Buskin visits LA's Modern Sound, home to a decade's sci-fi sound