To be useful to travelers and family members, flight trackers often have to fill in some of the gaps in the FAA data, gate times, for example. There are other variables, too. There is typically, but not always, a message sent to the FAA on departure. In the cases where that message is not sent, a flight tracker needs to make a best guess about the actual departure time. The same goes for arrival times. The methods that the various flight trackers use to guess vary causing discrepancies in information provided by different flight tracker tools.
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