Real air traffic is actually realtime, but close to the airport the AI takes over and communicates with ingame ATC following the active runway etc.
So you’ll see a discrepancy depending on the weather conditions in the sim.
Aircraft on ground are not rendered from live traffic, they are randomly spawned. Real traffic adds up just prior departure.
Not sure how your setup is but I always have plenty aircraft standing at the gates/parkings, but you won’t see any nameplates on them since they are inactive.
Also note that the number of real time (or active AI) traffic will be reduced if you have multiplayer aircraft in your vicinity. They have priority and there is an overall limit of 30 or so closest active aircraft rendered at any time.