'It's just normal MMR ecks dee' is just an idiotic way to say you don't know why this happens. Normal MMR is largely unimportant. The system prioritizes creating games fast and will completely ignore MMR if it can't quickly start a close game.
It also doesn't try to create games where everyone has similar MMR, it only tries to create games where the average MMR is close, and even then, it will still just grab whoever it can to get a game going fast. It makes no effort to keep everyone's MMR close, and if there are two premades in queue, it makes even less effort.
i.e. the system will throw two Diamond players onto a team with three Bronze players against a team of five Gold players and call it a fair match, because the MMR on both teams averages it out, regardless of the two Diamond players being miles ahead of everyone. Or if there's nobody playing, it will just throw the first 10 players it can find into a game.