I think I know, and this is just a guess that has to do with last year's writer's strike. Is it possible that season 6 was just cut down to 13 episodes?
According to production codes, we've got 11 S6 episodes aired of tonight, which would leave two. (I'm guessing)
Now, I believe I read somewhere that the next Star Wars special 'Something Something Dark Side' was intended to be the S7 finale because that's the way Seth wanted it. What if 'Dark Side' was the proper S7 premiere?
Maybe that's why we've got the delay, because they're saving that episode for the end of the season.