The service manual says that? That's a little high IMO, but in a pressurized system, the boiling point of water increases.
http://www.engineeringtoolbox.com/saturated-steam-properties-d_101.html
What is the pressure rating of your radiator cap?
http://www.engineeringtoolbox.com/saturated-steam-properties-d_101.html
What is the pressure rating of your radiator cap?