Template:Realworld Both the Spartan and Elite player models have differant hit boxes or collision models. This is an invisible box around the characters, hitting the box will count as hitting the visible portion of the character. This box extends beyond the visible part of a character, so firing just next to the head of a spartan or elite will count as a headshot.
The hit box is the reason that when standing off the edge of a wall, your character will "float" because the hit box is still resting on the wall.