Character mistake: Prior to the opening battle, Quintus says "The danger to the calvary" instead of "cavalry." (00:04:00)
Character mistake: When Maximus is talking to Marcus Aerelius, he mentions that his wife has black hair. But in the scenes where we see her, she actually has brown hair. (00:24:40)
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