[cookinwitdiesel]

Everything is designed with some safety margin built in. It is normally never published as that will invite people to push the margins, treat that as extra normal capacity, and break something.

This margin is there to handle things like shock from hitting a pothole or a bump in the road that bounces the trailer slightly.

To answer the example above, when the hitch is momentarily weightless, whatever popped up the back of the truck will almost immediately pop up the trailer axles as well thus "transferring" much more weight back onto the tongue. Is your tongue and hitch rated for many thousands of pounds? No, because they don't need to be, because a) when the tongue/hitch is "weightless" from a bounce the weight is balanced and not all shifting back on the axles and b) the same is true as the trailer comes back down, the weight does not all transfer to the tongue although momentarily that may be the only physical connection between the trailer and ground. Hence my statements above on axles ratings vs trailer weight.