Yes, yes, yes - rank and position are separate concepts. Hence my point that regardless of Kim's rank he'd still have the same position. So why promote him? He'd have no more responsibilities than he would as an ensign, and no more opportunities to demonstrate leadership capability, so why give him an extra pip? What's he going to do with it, get extra replicator rations?
the same applied to Tuvok. After his promotion he was still Security Chief/Tactical Officer and (presumably) Second Officer.
Sometimes promotions are given out of recognition for good work, not solely for greater responsibilities.