If you're below Diamond, it's based on your performances and stats. It also depends on the average SR of the teams, if you're on the team with higher average SR then you're expected to win according to the system. That makes you lose more SR incase of a loss.
I've realized that achieving consistent 2-3 gold medals I was losing about 10-15 sr per game and winning 20+, so it's probably on stats and winstreaks.