What is up with matchmaking?

I'm in promos and 3 of my team mates are gold but one is silver. The silver player was Morde top vs Gangplank. I'm not going to claim I know how that match up goes but he gave 3 kills within 5 minutes and he wasn't ganked. Now I'm not mad at the Morde but I don't understand why matchmaking isn't strictly based on your current rank. I'm not being sarcastic I'd really like to understand why matchmaking is like this?