my recollection is: that different [sized] ships have different port and/or passage fees (in the case of panama and/or suez canals) ... it's probably the same (by gross tonnage maybe??).
anyhow, those things could account for some disparity, as could whether the amount quoted includes gratuitous gratuities, and the 'stuff' cruiselines tack-on as 'services'.
might be a good idea to ask either your t/a or the cruiseline, to give you a breakdown of charges. seems like you're entitled to know what you're paying for. right?