Seems like smart bookmaking to me. I'm surprised more books don't restrict this.
Consider Minnesota playing Detroit. If the total is 5.5 you know that if it's under then Minnesota has a GREAT chance to cover. Why? Because the Star (like the Devils & others) rely on a tight checking neutral zone trap style of play whereas the Wings like to attack.
Minnesota is going to with the 2-1, 1-0, 3-2, because these scores mean that they played their style of hockey (ie, they dominated). Detroit will win the 4-3, 5-2, etc .. games more often than not.
Sure, Minnesota can put up 4 now and then, but most often than not they won't. It's not how their system is drawn up.