I think you're being biased way too much about it, it has nothing to do with personal skill level, it has to do with the amount of games played together.
I've played with a rogue called Lionhearth since s4, and in S8 we beat every single team on S8 TR that qualified for the regionals ( as RPS ), We had positive or equal scores against Enigmz LSD, Hydra RMP, etc. Then I've tried with a multiple r1 rogue who is supposed to be much better than my Rogue, and we fucking failed miserably, we played like 400 MMR below the MMR we played on with my previous rogue. The Rogue played just fine, I played just fine and my healer played just fine, it just didn't work as well as it did with my previous Rogue because we just didn't have shitloads of games together to compete on the top level.
Same goes for your buddy volkovitch or w/e your DK's name is. You probably by now have shitloads of games together, and if you replace him with a equally good death knight, you're going to see different results mostly because new DK is doing things differently. Just his tone of voice and his decision to go around which side of pillar is something simple enough yet different compared to your current DK.
It's like changing your wife for her twin sister, they'll be remarkably the same, yet they'll do things differently. And I'm not going to say which things differently because it's going to be a sexist remark no matter what I mention