It's because of the localization. The translations of Japanese to English for Pokemon, Items, Moves, characters, region towns/landmarks, and so forth is already done.
All they will need to do is translate the new dialogue and a few other things, then the game will be ready for release. That's why there is such a short turnaround this time.
Also:
http://bulbanews.bulbagarden.net/wik...begins_to_leak
http://bulbanews.bulbagarden.net/wik...agazine_leaked