I'm talking about the center of the reticule being locked onto their head with the secondary radius extending past their head. In these instances there is a slight chance the game will decide that the bullet will travel along the periphery of that circle and miss the player. Other times, I'll be pointing the center of the reticule half way down their chest or maybe to the side of their head, but the bullet travels to a random point within a greater circle, many times netting me a headshot when I clearly wasn't aiming at their head. My roommate and I had a good laugh the other day when he booted up the game for the first time, he complained the pistol wasn't accurate enough, then seconds latter he got a headshot with it when a player sprinted past him. We both laughed because his reticule was clearly no where near head-level let a lone on the players head.
This randomness is why you'll often hear me gleefully cheer out "Yippee, the game gave me a headshot. The Halo Gods are too nice to me."
In Halo CE you never got headshots unless you were aiming at the player's head but in Reach it happens often as well as not netting headshots when your aim is actually true.