So you have the Decurion, who will have years of service and training under his belt and presumably previous combat experience, but also has to control his horse with just this thighs when he has his shield in one hand and his Spatha sword in the other.
Where as our Queen of the Iceni here, seems to be quite steady and comfortable standing as she rides along in that chariot running at speed can concentrate on using her weapons to their best advantage and as Rob says, she does have the longer reach !
But a jolt to the chariot going over a stone say or any of the horses having a mishap....Mmmm, yep it's in the laps of the Gods. Let the Dice fly !{sm3}