The Nash studio model was only seen docking with DS9 "upside-down", at least in respect to the modification as shot previously as the USS Jenolan in TNG: "Relics". This was done by producers either for aesthetics, or to create the illusion that this was a different type of ship. Since it was plainly visible as the Sydney-class model, we should assume that the ships are the same class. The reason for the upside-down flight could either be that there was no correct "up" orientation in space, so it didn't matter that the Nash pilots continuously dock as so, or possibly that the 24th century Sydney-class vessels are modified in a way that that they are rearranged to have the opposite internal structure. It was even possible that, since the Jenolan was only seen once and was in pretty bad shape at that time, that the Nash is the one being flown right-side-up.