The route is not transmitted from the aircraft. We are receiving the call sign of the flight from the aircraft and
comparing it with large databases of airline and airport schedules to find the matching flight number. Once the call sign is matched to a flight number, we are able to show the route of the flight. A few reasons why a route may be incorrect:
Changed flight plans
Wrong or old call sign typed into the transponder
Long flight delay, which makes the schedule data match against the wrong flight
Error in some of the schedule databases