it's because ppl think: more damage = max contribution...which is not the case.

i did a low lv map, and burst down mobs, and got bronze medal...and i was soloing the map.
i did same map again, and auto attack, and get gold.
the higher damage everyone does, the less contribution, everyone share.
so if everyone was spamming weak skills, everyone would get gold - but nope, ppl want to use LB on FATE bosses, and hunts.