I don't know if I can answer this well enough but I will try. The weight rating for tongue weight is different from the accessory because in the case of the tongue weight, it is from a trailer being connected. the weight on the hitch can vary depending how the trailer is loaded. If more weight is toward the front of the trailer, there is more weight on the hitch. But in all conditions, the weight on the hitch is always mainly straight down. Because weight is distributed between the hitch and the wheels of the trailer.
In the case of an accessory being connected to the hitch, all the weight from the accessory is being supported by the hitch, so you have a weight on the hitch that is cantilevered (hung) from it. The hitch is not only resisting downward weight, but also torque. The accessory hanging from the hitch is trying to bend it (twist it) downward. It is this torque that has to be accounted for and therefore reduces how much weight the hitch can take versus a trailer where the hitch only needs to oppose a downward force with little torque.
If we have any mechanical engineers, or perhaps structural engineers in the audience, they can explain it better than me. But there is a physical reason why the rating for an accessory load is lower than the rating for a trailer load (tongue weight). It's the same hitch is both cases, but it is being loaded differently.