There IS a simple answer, Matej.
I have seen AAL flights from Europe to KDFW on my area map, showing location, ground speed, and altitude, yet when looking for that specific flight, the flight history indicates it is not yet en route. I know, however, that it is about to land.
The flights show up on the maps because ATC is tracking them on radar. However, the overseas aviation authorities (over whom FA has no control) has not yet issued a ‘departure notice’. Therefore, “flight data” does not yet exist.
Evidently, in closing out the flight after landing, the FAA creates the departure notice along with the arrival notice. At that point, FlightAware gets the historical flight position information.
It’s a screwy situation, but FlightAware has positional information only after the FAA issues the data.