At any given time, the weight of the truck is hopefully carried by its wheels and nothing else. If I weigh them with individual scales at the same time, the sum of the weights must necessarily add up to the total vehicle weight. If there is a difference, there was an error made somewhere in the weighing process. I do not doubt that the displayed values don't always add up for whatever reason. But surely the combined axles carry the combined weight of the truck, no?
Not the weight police either here either. You can cheat on many laws, but not the laws of physics.
Whatever weights and ratings... if it handles well, it can not be too far off. Just be aware of your tire loads.