Duo Q vs match making - Too high of a rank for duo but doesn't effect randoms?

Was wondering why this is in effect. Had some bad games in 3's, happens to everyone. I assume the rank of players is based on how low i've dropped in my elo in a sense.

Now you can't duo with someone if the ranks are way too far apart (I'm im Plat but I can't duo with silver) but I can get matched up with people in silver ranking in that queue. How come one is allowed and not the other?