The F150 has natural rake with the rear end being higher to give a better ride with more flex. Once it gets down a bit, it stiffens up and acts more like a typical truck spring than a car spring.
So when the truck's loaded, it will carry a natural equal leveling of the front to the rear. This is most likely the reason why Ford doesn't want the front level being as low as when not loaded. Keeping the front end higher will keep the rear lower than it's natural unloaded height, keeping the spring in it's stiffer range.
That being said, I believe the OP's truck squats a bit to much in the rear meaning that more weight does need to be transferred to the front. This will likely keep the rear axle from overloading until the GVWR of the truck is met, as I've seen with others with this generation (2009 - 2014) truck.
