Whenever you start comparing the information from multiple sources you will inevitably see differences. This happens because:
- There is no central source of flight data, which means different sites may be getting their information from different sources.
- When data is made available it will not reach all the different sites at the same time.
- You may not be making an apples to apples comparison. Many sites only use runway times, FlightStats uses both gate times and runway times.
- Sometimes site A will not have access to the same information as site B for a particular flight.