Sounds like they're playing the blame apportioning game. But even then, there certainly seems to be a problem with their maths.

If you are confident you didn't contribute to the cause of the accident (other than unfortunatley being there), my suggestion:
Don't sign acceptance of their offer (that's all it is).
Respond by instructing an attorney to issue summons for the full quantum of your damages against the driver (and/or owner if the vehicle was being used for business) of the other vehicle.