Hasn't he led in that statistic or at least been close for the prior two seasons as well.
I realize that a lot of people expect him to be a big hitter, but in that case his size works against him. If he throws a big check the referee's will call him for something, probably elbowing (sound familiar?) On the other hand that size differential doesn't work for a player like Gennoway when he takes a check from a larger player.
But the bottom line is that he plays his defensive position well more in screening out the players and depucking them with his reach. On offense he makes good decisions with the skills he has.
Bingo, big leader in plus/minus. It's not a fluke over the course of a season.