To start, the last 4 seasons combined, Subban has significantly outplayed is team on the penalty kill averaging 5.66 goals per hour against compared to his team's 6.2 and averaged 46.6 shots against per hour to his team's 49.7
Montreal has been the 7th penalty killing team in the league during this time frame, so its not like he's outcompeting scrub competition.
In comparison to the entire league, Subban has the 7th best shots against rate and 12th best goals against rate amoung defenseman who played at least 400 minutes of penalty kill over this time frame.
Neither Burns, Pietroangelo, Weber, nor Doughty have a penalty kill stat line remotely as good as Subban's. Pietroangelo has pretty good overall results, but they are in the context of the league's best penalty killing team and Barret Jackman is the star performer of that group not AP.
At even strength, Subban has run a goal percentage of 54.4% which is 4.76% better than his team over this stretch of years. He is likewise a 4.18% better than his team at corsi%. His corsi for is best and against is the 2nd best on his team for these 4 seasons. Subban runs in at the 12 best relative goal percentage in the league better than any of his Team Canada comparison players and 13th best in relative fenwick percentage amoung players with 3000 minutes (roughly top four defenseman) during this time frame.
In short, Subban's statistical tendancy is to massively outperform his team at puck possession at even strength, which results in a commesurate out-performance in terms of results.
Compare this to both Pietroangelo and Weber, who haven't outperformed their team much in recent years, and Subban looks like very much the superior even strength player as well.