Terminals are the most likely cause of a disconnect between the gates available and the gates used that you can see, however, there is a chance that a small discrepancy comes from new accounts joining there. The game creates new gates for new airlines when none are available, and it only skims gates back when the airport has a demonstrated availability of over 110% of the normal gates for the game at the month end. as that airport is the first in the list, it tends to see a little extra attention from multi's, so this could actually be the case.
