I bet NoA's marketing department crunched the numbers and determined that an October 3rd release date will be more lucrative than September 13. As for why, I can only guess. Maybe it has something to do with back-to-school season: having just spent a bunch of money on school supplies, parents may be less willing to drop another $40 for a video game; but by October, enough time has passed that school expenses have drifted back a ways in their minds.
Late, but that was always my guess in regards to release date.
I just don't get why they're so against releasing a demo for it ahead of time, assuming they actually do.