Tire ratings / axle ratings are just like Over Head Crane ratings..
They are rated to a certain rating with a safety factor on top of that.. They POST the rate based on every day usage.. Most people will see that they can carry XXX amount of weight and go full Hog till they cant..
I can assure you, If they rated our overhead cranes to 15 ton instead of 10 ton, we would be lifting 15T every day, all day... in a nut shell - There is a safety factor built into EVERYTHING with a MAX capacity.. just how much = ??