What's up with matchmaking?

So I was just playing a normal trying to grind out lvl 7 on Yorick and I had an upsetting surprise. I loaded into the game and I saw that my team had a diamond border, gold border, and plat border (the last person was either gold/plat, I can't remember). Now this seems normal, until you notice that I am Silver 4 (in my promos, a little nervous). I was on a winning streak, but I still wonder why I was put with someone who was in DIAMOND. That is just ridiculous to me, if you are in x rank and on a winning streak, you should be put with people one tier above you, not 3, that is just ridiculous.