Why does "Depart 20" only depart a few planes when I have many ready?

The bulk departure tool selects up to 20 aircraft from your list before checking their maintenance status. If any selected aircraft are currently in maintenance or awaiting an A-Check, the system skips them during departure.

Example:
If you click Depart 20 and 16 of the selected aircraft require maintenance, only the remaining 4 available aircraft will depart.