Unfortunately, slots are always filled first come, first served. This is to avoid favoritism and to avoid DMs hand selecting their rosters. This won't change.
It has been explicitly said that the policy is to try and prioritize people who get into quests less, and I have seen DMs do this in practice. So that is at odds with that statement.
My complaint is based on being told that. If it is just first come first serve, then so be it, but contrary statements about this have been made, and so there is confusion.