1. Send unit from planet 'A'  to Coordinates 'B' via defense. 
-The unit will always carry enough fuel to get to destination, hold for the chosen amount of time and return to starting point.
2. Unit arrives at Coordinates 'B'

problem
3. Before the duration of defense is up, select 'sending' and deploy to Planet 'A' (the starting point)
-Why does this cost more fuel than a defense loitering on target longer + its return flight to Planet 'A'?

This makes no sense. I could understand if you were sending to a target other than it's origin point before sending it home.




Also

Deploy
When deploying it states a return time. There isn't a return.
Also When deploying it will always carry more fuel than needed to reach the target.

---It would appear all deploy missions are being treated like 'Transport" for the sake of calculating fuel expenses.

Mission deploy provides the possibility of returning, that is why it takes fuel to the both sides. Do not choose deploy mission for the returning if you don't have enough fuel, better choose simple returnig without deploy.
It is not a bug.