Just like the other players in a match affect your win rate, they also affect your statistical performance. Blizzard doesn’t have any direct way to observe the true ability of a player. All information is indirect. However, over many games (several hundred), the only consistent factor is you, and your rank is reasonably reliable.
Yes, stats can also cause people to chase stats rather than wins. This is why performance SR was removed above plat.
Blizzard hasn’t published anything like a formula on how performance SR works, just some fairly vague rules of thumb. See that How Competitive Skill Rating Works (Season 13) → Performance modifier for details.