The balancer is responsible for selecting players. The balancer "checks" your highest tier tank and amount of battles on this tank. It's set as priority for balancing. So if 2-tier and 5-tier tanks meet in the battle it means that:
a) 2-tier tank has many battles and isn't considered as newbie
b) 5-tier tank has few battles and isn't upgraded
c) for some reason players take lower tank having higher tanks in the hangar
Moreover, some tank levels are united into one tank group.