What CPRich says is correct. It is also possible that there are some preferred seats that are not yet selected that you can't 'see' on the seatmap because you have to have status to see them.

I believe the folks around here have figured out that if you go on the USAirways website at 2 AM Arizona time the day of your flight (I am sorry to say I am not making this up) the full set of available seats is displayed, including the ones you can't see or select at this time.