How to receive a head injury with the helmet on?
When the IED goes boom, you go flying around inside your vehicle like a rag doll. If not properly strapped to the chin, the helmet can easily be dislodged. Or, something inside the vehicle (e.g., tire jack) could easily have become unsecured and whacked him in the noggin. Many scenarios.
Explosion(s), then small arms fire. Really only protects the crown, upper brow, and some of the ear. If they were above the explosion(s), which is likely, it is amazing that they were not wounded more grievously.
LOL< easy those kevlar helmets suck!
Read this from the US Army:
http://www4.army.mil/ocpa/print.php?story_id_key=8498
Note that the kevlar was shot with a glacing blow, yet still penetrated the material. If I can find the picture I will post it.