Revealing mistake: In the opening battle, it's quite obvious in close-up the arrows shot by the ballistae (war machines) can't really be travelling more than a few yards at most, not the great distance they're supposed to be flying and which real ballistae could manage.
Continuity mistake: In the Gladiatorial fight scene in Morocco, Maximus throws one sword with his right hand into the stand, still holding a sword in his left hand. The next instant long shot of him shows the remaining sword in his right hand as he taunts the crowd with "Are you not entertained?"
Chosen answer: Because they would be the ranking soldiers, and accustomed to being given orders. Being disciplined, they would obey Quintus without question. The legions would necessarily have soldiers assigned to following orders at different times, similar to how a captain would give an order to a full room, and the first mate would be the one to carry out the order.
rswarrior