The limbs don't really fall of from shooting them in a direct sense. It is more a response from how much damage is inflicted on the zombies. Example: Shooting a zombie in the body without causing it enough damage to kill it will cause it's arms to fall off. So essentially shooting a zombie anywhere except the head will grant gibs. It's not a glitch, that is just how the game is designed.
If you have ever played Arkham City, Batman's suit get damaged more and more as you progress through the story. This is pretty much the same just at a much more rapid rate of change. Want a lot of gibs? Lay down a claymore on a hoard on a high round. It won't kill them, but there will be arms all over the place.