Everything wrong with SBMM
Me and my mates who are Grade Iridescent 1 and are really experienced in DBD have just been against a Doctor who was Grade Ash 4. I feel really bad for him. We have won lots of games and we still have the same MMR as this doctor for some reason? SBMM is just a joke

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When you loaded in the lobby you probably matched a killer with suitable MMR who dodged and then the system just grabbed this guy as a replacement. From the most recent dev update it seems they're looking to improve this.
Today's Developer Update Part 1 talks in some detail about issues with the matchmaking they're working on addressing. Long story short though a lot of mismatches happen because:
The update mentions changes they're working on regarding how long the system waits for close matches and lowering the starting rating of new players. They also are giving solo survivors a small boost soon in the form of extra information on the HUD about what the other survivors are doing. (Basically an icon with a gen or health pack or running or a totem, etc, so you have an idea what they're up to.) That should help even the playing field for solo players against swf players without giving the swf players a boost. Hypothetically if it turns out that this extra information makes survivors escape a bit too often they'd probably just slow gens down slightly to compensate is my guess.