Probably the same reason IL-2 had it's rating. Guy bails out, you shoot him and he goes limp (dead), Gunner/Pilot gets shot and he goes limp (dead). Only reason it's 16+ is that the guy looks a heck of a lot more realistic than he did in IL-2 so it seems more violent.

Another thing might be people being stafed on the ground, or vehicles/planes exploding. You don't see someone being killed, but you know that's what just happened.

Or it could have just been something tacked on while the rating is pending?