The vault was ornamented with coffers. Some triumphal arches were surmounted by a statue or a currus triumphalis, a group of statues depicting the emperor or general in a quadriga. The inscriptions on Roman triumphal arches were works of art in themselves, with very finely cut, sometimes gilded letters.
In my opinion, its what the artist is trying to write ''story wise''. Kind of like a comic book accept done all pretty. I can't tell you a ''correct'' answer of this type of question but i hope you get the main idea.
