The seating chart on USAirways.com *usually* shows fewer seats than are actually available, but as always it depends.

The employee site will show the number of unbooked/unpaid seats as the number of available seats. When you look at USAirways.com, you'll usually see fewer seats available than what the employee site shows because it is showing seats that are booked, but not yet paid for, as occupied. At least thats what I've been told.

Within an hour of departure is when you get the best accuracy on either site, naturally.