Maybe because Valve is based on the Pacific coast?
That doesn't answer the question
Direct2drive is based in California (Pacific coast), and they are releasing the game at 12:01am PST, but Valve is waiting till 11:00 AM PST on the 17th, 11 hours past the World Wide launch time. Valve being on the pacific coast has nothing to do with why they are releasing the game 11 hours past everyone else.
I beleive we can apply "Valve time" here, even if this game is not developed by that company