I consider one of the few plausible explanations that the game concludes that I, generally, win in spite of myself on the basis of my stats. But my, for example, Pharah play isn't going to be improved by padding my stats by firing rockets into barriers, which again comes back to the notion that the system is not perfect and is often less than adequate as a predictor or skill.Just was taking a guess based on what you were talking about happening. I'm pretty confident in talking about the SR system because I've messed with three accounts now to see how some of it works, and I'm friends with some boosters who talk about manipulating it for profit. It's murky, but in general it's pretty easy to predict what's going on with an account and reverse that MMR issue you have. It almost always comes down to long strings off subpar stats.
Note: I am not a booster, but I have looked pretty deep into it. SR manipulation is a real thing people have figured out. Pick Torb or Sombra in a stack, farm specific stats, gain 150SR a game.
If the system went off of my win rate alone it would have pushed me a lot further upward because even at 3900 I had not reached equilibrium and was winning more than half of my games. It's not doing that, however, and it's not doing that to the extent that I'm really getting matched with/against people I have no business being matched with.
I've considered getting a smurf to see if I can replicate these results sheerly on the basis of how many commonly-held assumptions about the SR system (bias against supports, bias towards positive win rate, etc.) my current situation contradicts.