You know, promotions take a good amount of time anyway. For the amount of years humanoids seem to live in the Trek universe, 7 years is a drop in the bucket as far as time goes. For all we know, it could take a decade before one is even considered for a promotion.
Tell that to Chris Pine's Kirk.
Or, for a less totally insane example, I think Nog ended DS9 as a lieutenant, five years after starting the academy.
Indeed, also theres Geordi LaForge who went from Lieutenant Junior Grade to Lieutenant to Lieutenant Commander in two years(!)
Course, Janeway had as many personalities as I have socks... so it could be multiple reasons.