It's these kinds of outliers (vs the team average) that give the stat meaning. Obviously, a significant sample size is a must, as so many +'s and -'s have nothing to do with individual play, but a trend like this that adds up to Kunitz and Niskanen being so far ahead of their teammates indicates what it's meant to indicate: they're playing well, good things happen when they're on the ice. Any small variations in +/- are pretty much meaningless though.
Another interpretation is that these players are enjoying a lucky streak (they do have the highest PDO, another advanced stat meant to indicate "luck"). But either way, it means that they're enjoying success right now, so ride the wave.
In Niskanen's case, he has improved significantly in each of his season's here. Luck, for him, could be explained by the old adage of preparation + opportunity. It's hard to gauge what his ceiling is, and it should be interesting to see what Shero will do when Scuderi returns, as Niskanen, Bortuzzo, Maatta have all been exceptional.