Unit Experiance

[Unferth of Gergovia]

Hey everyone...Ive noticed unusual experience gains with my group of Gaesatae in Gergovia....Ive only fought a few battles with them and some Bataroes but they are all either 2-3 silver chevrons or 1 or more gold chevrons....they had the fastest experience gains of any unit Ive had...is there a reason for this or is it random and what are the criteria for experience gains...I would love more info so I could fine tune my legionares at some point