Everything in the world has a factor of safety engineered in. If not, the elevator at your office would fail at 2401#, the floor in your house would fall through when aunt Sally tilted all her weight to one leg of her chair and a whole bunch of other bad things would happen.
And, in truck terms, it's because we humans are too dumb for our own good. If every 3/4 ton truck exploded at 1# over its GVWR? Well, at least the highways would be more open on Sunday afternoons.
So, how much of a factor? Before dropping out of Engineering school, I seem to remember a 50% over was somewhat standard on things that REALLY mattered. Like elevators and bridges. A truck axle or other truck weights? Who knows. But, it sure is fun watching those that think they do :)