sgt203,
You are thinking in a real world traffic, at fault, manner.
You need to put that aside and look at it like a computer.
The computer doesn't care at all who was going which way or what parts collided. All the computer cares about is if there was a collision. On your system there was a collision. On his system there was not. Hence, you collided with him. No fault, just a statement of which person's front end detected a collision.