I find that even if you are victorious in fighting off the sally your siege is broken. This allows all the free units in the city to auto respawn.

In all other TW games if the defenders tried that they would automatically lose the city on defeat. I am betting this is still how it is supposed to work. You win win the town so the siege is not necessary but since you are never allowed to take the city the defenders respawn just like you had been beaten.