Well availability issues do happen from time to time. However from my window I just witnessed an inbound tram from Alty running as a single and full with several people standing, whilst an airport service runs out of the city centre as a double. This is a bad allocation of resources surely! I know that not all sets can run to Alty but around 70 can, and only 16 would e required to run as doubles every 10 minutes!
How do you know that Airport set isn’t one that gets busy later on? Or early in the morning? The doubles on the airports were deliberately targeted to operate the busiest services on that line. It’s not as simple as you think I’m afraid.
You can’t rob Peter to pay Paul.