I think the September 15 date is a red herring, at best a placeholder for the merchant's software.
Canon hasn't given a specific date in September, so some merchants are hedging their bets and putting in 9/15, since their software probably can't hold just a month and year. That gives them one shot of it being "right", 15 shots of it being "late", and 14 shots at it being "early". They could have as easily put in 9/30, and then they could only have happy customers, since there would be 29 days it would be "early", yet still be in September!
The other sign that, to me, indicates the September 15 date is just wrong, is that September 15 is on a Sunday. Most product releases would happen during the workweek, often on a Tuesday, and almost never on a weekend.