Why are hero SR rank and actual rank different?

So I used to be a plat OW1 console player - I don't say this to imply I'm any good so no need to flame me, I know console is obviously incredibly different and I don't believe I belong in PC plat AT ALL, and to top it off I have had a very long break from comp. I got a gaming PC last year and recently started learning KBM, so my aim is only just getting to a similar point to what it was on controller, however I do have basic game sense as a tank/support player. I'm pretty sure my PC QP MMR absolutely tanked while tweaking my sensitivity settings and learning how to aim, I was genuinely quite horrific at first. I'm ok now and honestly don't play a lot of heroes that need good aim anyway.

So I recently did my placements for the first time on PC and immediately was being placed with bronze players - which is fine, and I don't think I'm "hard stuck" in bronze, I am on the whole winning more than losing but it's so slow and sometimes there really isn't much you can do if no one works together and decides to run straight into a bastion. I would really love to find some people to queue up and improve with but it's kind of embarrassing to say I'm bronze and I figure from the chat most people down here are actual children.

But I've mostly been maining Dva/Domina and the game is saying my SR is mid-silver for both heroes, and yet I can't get past bronze 1/2. It's really quite painful down here. I think low/mid silver is a good place to aim for, and half the time in my games I'm literally playing against and killing silvers or golds bc the matchmaking is so weird. I just don't really get how the game's SR system and ranking system don't line up if they're both based on the same metrics?