I guess we traded teammates then, which is even worse, because it means it's averaging MMRs by simply rearranging teams.
Well, to be more accurate, it's rearranging teams after adding in new players. It takes those players and then creates a new match based on their current MMR, without taking any regard to the last game. The Garen and Lee didn't switch teams just because one won and the other lost. That's irrelevant to the decision process. The only reasons that they're in the same game as you again is because they queued up at the same time, they're at a pretty similar MMR to you, and they're also in their placement matches. I don't really see why this is a problem. It's pretty standard matchmaking.
A +/- of 100 MMR is enormous.
Well, again, that's not what it is because that disregards all the new players added into the equation. 6 of the 10 players were new to that game. You didn't play a 100 MMR difference game. You played a pretty even MMR game that happened to have a few of the same players who were in an earlier game with you.
And the thing is, I'm not climbing the ladder.
My point with this was that you're in a place of system uncertainty. You can't say that your MMR is lower than theirs, because you don't have a well-established rank. That claim of yours was false.
Look at the CS of those games. The score. The tower count. Dragon. Baron. I am not at that MMR. We are not the same. That means the system is either A) bad B) deliberate or C) both.
If you're not the same, then you'll win more often than they will. That doesn't mean you'll win every game, and you're not expected to unless you are a very skilled player. You shouldn't expect to either. The system is matching you with other people in their placement games and with people around silver level, which seems to be roughly where it expects you to be. If you're better, you'll win more often and you'll rise. There's nothing more to it.
Also I'm not taking any conclusions from two games unless I can actually see the players play. I'm not going to believe or disbelive you on this. It's just not anywhere near enough to actually make a conclusion on.