Actually, they don't face similar quality of competition. I posted about that last week. Muzzin has been 4th in QoC among defensemen on the team this season. It's noticeably lower than his usual defense partner, Martinez. Are you carrying your pairing when you face lower quality competition? He did have noticeably higher QoC than his partners the two previous years, but he also played a lot worse. It seems like getting easier minutes this year has really benefited him (i.e. he's the one benefiting, not Doughty)
You're right. I shouldn't have called them his career numbers. I meant that those are his stats since 2014-15, his first regular season as a top-4 defenseman. He was a +18 and a +11 in his rookie and sophomore years in 2012-13 and 2013-14, respectively, both played on the 3rd pairing. You were right to correct me, but I don't think that those seasons are really relevant to discussion of whether he's a top-4 (let alone, top pairing) defenseman.
Muzzin contributed to only one Cup win.