Most often, when a game is just giving a general time frame (like a particular month or "fall"), stores will usually put it to a specific correlating date. For example when they say "October" but don't give a specific date, the stores will usually list it as being released on October 1st. They will then change it to the official date once it is released. While I don't know the exact reason why companies do this, I always assume it is because their databases only have specific dates (not general times) for the release date field. In other words, they can only list their releases as mm/dd/yy, not "Fall 2010".
In this case, I think Steam did pick the wrong date to put in there because, though September is the first month of Fall, the date that they picked is still technically Summer since Fall doesn't start until the equinox, around the 21st.
I think a more likely date for release (assuming it holds to a fall launch, and, if I am not mistaken, Firaxes is usually pretty good on there dates) would be around the end of October or early November. At the latest I would see it being released would be just before Thanksgiving.
