Joined Trill and Starfleet Rank Carryover

[TCGM]

As we see in Deep Space Nine, the various Dax-host Joined Trill seem to all hold different ranks in Starfleet. My question is... Why? Shouldn't the rank achieved by any given symbiote/host pair carry over to the next pair, as the symbiote retains all its training and experience? Or at the very least, the new rank should be the split difference between the pre-host humanoid Trill and the symbiote's rank.