I believe they re-use the number. A few months ago (maybe almost a year ago), about 20 minutes before the flight I was at the desk waiting for a boarding pass and the lady in front of me in line got an "A" pass. The gate agent said something like "You lucked out, somebody must have changed flights or cancelled their flight last minute so you get their "A" number". I was next in line and got a "C" on a pretty packed flight.