American will mark a flight "awaiting take off" once the door closes but for various reasons the plane might sit at the gate for another 5-30 minutes. Then that time is updated again once the plane finally begins to push off the gate. Accordingly, the plane might not be actually "awaiting take off" but still at the gate. As other have noted there's 5 to 15 minutes of activity that needs to occur after the last passenger has boarded.