Matchmaking???

So let me start this by saying what I think I know about dynamic queue, if there's a team with a duo or more people playing together, their collective MMR is supposed to be lower than the collective MMR of the enemy team, right?

Now, in one of my games, we had a plat 5 duoing with a gold 5, so far so good but then... there's a silver on my team? silver 1 at least but you could tell that this person was FAR worse than anyone on that game, obviously he was in a duo with a gold 2 player and we also had a plat 5. The whole thing looked like this:

Red team (mine)--------------------------------------Blue team Plat 5-----------------------------------------------------------Plat 5 (duo) Gold 2 (duo)----------------------------------------------Gold 5 (duo) Silver 1 (duo)---------------------------------------------Gold 4 (promo) Gold 5----------------------------------------------------------Gold 4 Gold 5 (me, promo)---------------------------------Gold 3

So... uh I think that collectively blue team had an overall higher MMR than us no matter what, the gold 5 on their team wasn't playing Leblanc at a silver level either way so I was seriously surprised about her ranking when I saw it afterwards...

I'm not really salty about that game because overall I did what I had to do but the whole matchmaking thing looks fishy, don't you think?