How is this matchmaking fair?

So i am leveling my second account so i can play with my brother who is in bronze because i am gold. For some reason the servers believe that my account has the mmr of a high plat/diamond player so i keep getting matched with those players when my entire team is gold/silver. why is it that i am being matched against such good players when i don't even have a full rune page and a full mastery page? it makes no sense.