As you can see from the image above, this player won 9 games and only gained 45 points. Then, he lost 1 game, immediately erasing 45 points from his MMR. So if you win games, you get very little points. However, when you lose 1 game, you get penalized so much. Actually I have no idea how Valve handles the MMR points reward system. I guess they ought to at least explain the reasoning behind the difference in points given for different games. Are they much harder? Or…we never know.
Image Source: Bakri Anwar