I don't understand SR gains.

In 25 games as winston I've won 70 percent of them and am 50 sr lower from where I started. This honestly just makes no sense to me. Why do I have to remain where I am when I am clearly better than the other players around me, and why do mercy mains with below 50 percent win rate climb? If this was fair shouldn't it be 50 percent win rate stays where you are? I got 3 sr from a fifteen minute game earlier. Don't really see any way that can be justified.