the game pulls available aircraft that ‘fit the mold’ for w/e parking spots are available
so if you only have default aircraft things are kinda balanced, but if you have some addon aircraft with lots of liveries the game will use each livery as a separate selection - that can give that addon aircraft more possible selections than every other similar aircraft all put together
there are some switches in the aircraft.cfg to enable/disable for ai and air traffic, but im not certain they disable static parked aircraft
not to mention the livery dev probably likes them showing up often - it shows off his/her work so they likely dont change them (0 is disabled / 1 is enabled)
isAirTraffic=0
canBeUsedByAITraffic=0