I can't recall this ever happening before.
I think the cancellation reason (more phone availability) is more convincing.
Actually the same thing happened to me during the iphone 4 launch. Preordered, got a 2 week delay quoted but it ended up coming a week early.
Same thing happened this time. Was originally given an October 5th date but I now have a shipping notification saying the 28th, so it has moved up a week.
It could be some cancellations, could be issues with payment's not going through, but personally I think they push the date back two weeks as soon as the initial stock is gone. As production is continuous some of those delays get shortened.