It could also be that the combination of lower muzzle velocity due to a much shorter barrel with a heavier bullet and the lack of expanding projectiles result in less likely immediate incapicatation. I've noticed that the deer don't drop with a chest shot nearly as quickly when using heavy bullets in 16" carbines. It's more like shooting them with a .308 Win. Effective, but not borderline frightening as with the 55gr in a 1-12 twist 20" bbl.
Mind you I don't think a .223 is a great long range deer round. But that's not a problem in the woods!