Okay, post graduation from the Academy. That makes more sense. Still was promoted twice... once to Lt. jg, and again to become full Lt. (This timing is very close, though, as it just turns 2365 when season 2 begins. His promotion happened between seasons, so there is room to say only one promotion.) However, the Lt. Cmdr. ceiling is very likely except for one thing... Sisko put Eddington up for promotion before he went rogue, so it shows that such a thing is possible. In his case, being in charge of Starfleet security on such a large station likely would account for that.