It's a function of numerous reasons, the most important one possibly being the limited number of snaps our guys have received due to injury or planned rotation. Notice how Chillar and Pop are close to topping 63 tackles. Each would have done so had they not shared time. If Bigby had played all 12 games, could he have been at that number? How about Barnett? Nick Collins?
We're also plus on time of possession, meaning our defense is on the field less than other defenses, which equals fewer opportunities.
It really doesn't mean anything.