From what I know, if you have subpar stats but start to rise, things get weird. Things get weirder if you're in a group because that influences SR gains quite a bit. Again, I'm not saying you're bad. What I'm saying is if you're picking popular characters, switching often, and not padding stats, your MMR isn't going to go up while your SR does. MMR gets more stable with more games, SR just gets more inflated.
The best way to get out of this btw is to pick unpopular characters and boost stats on them. The most popular ones to do this on were Torb/Symm/Sombra because they can be good, easy to p[ad stats on, and you can get far above average on them.
The proof of this I've seen is people making new accounts and getting 4500SR using mostly Torb and all that. I've seen people duo queue boost using Symm quite a bit too. It's why I complain about the SR system being too easy. If you have the skill to get away with things like that, you can get crazy high. Start doing this shit while feeding that group queue Mercy main extra stats while you're at it. Queue at optimal times with large groups for good measure.