That may very well be the case. After all, the main reason Tom Paris was re-promoted to Lieutenant was because after he was demoted to Ensign in Thirty Days, the writers still kept writing him as "Lieutenant Paris" in the scripts. Eventually, they got sick of having to go back and edit all references to "Lieutenant Paris" to "Ensign Paris" so they just decided to say "screw it, we'll make him a Lieutenant again."It seems most likely, people running the show just really loved calling him Ensign Kim.
Of course, that falls apart after you take into consideration they apparently remembered to switch from "Lieutenant Tuvok" to "Commander Tuvok" in the fourth season.