Because this same, exact footage was shown at Wizard World Con awhile back, with attendees saying that it was from the end of the film and Nolan hinting on further adventures for Batman as he did with the Joker card.
Who is to say that the car scene takes places BEFORE this one? I think it's a bit ridiculous to say whether or not this is definitively the ending of the film. I am leaning towards no, but I'm certainly not sure - nobody is.