FlightAware, as I said, gets its data from the FAA’s database. This data are pilot dependent.
As an example, take a look at N154C’s flight on 28 Apr 06. It used the code of “GLF.” All of the other flights used “GLF2.”
One way you can tell if a pilot used a non-standard aircraft code is to look at the flight information box. For the referenced flight, it shows only the aircraft code: (GLF/A). If it had been a code that is in the FAA database, then the aircraft type would have preceeded the aircraft code: Gulfstream American Gulfstream 2 (twin-jet) (GLF2/Q).
Pilots, unlike computer geeks, are not gods, unless they are also computer geeks, in which case they are only god 2nd class because of their pilotness.
Edit: RobK, you must have been replying when I was writing my reply.