Some routes shown - some not. Is there any system?

Hello everybody,

I really enjoy the service of flightaware. Mainly to fly real routes in my simulator. I noticed that all transatlantic flights eastbound are shown, but only some westbound flights. I guess it’s because flightaware covers North America. But what is the system behind the westbound trans atlantic flights shown or not? For example LH444 from Frankfurt to Altanta is shown, but Austrian 71 from Vienna to Toronto not.

Best regards