That doesn't mean that the Prom does variable damage according to target. It means other weapons should do it.
So the Prom can adjust its frequency according to the target to inflict maximum amount of damage. I suppose that damage is limited by the power of the "argon laser". Thus the Prom should inflict the same amount of damage to all targets. This on the other hand means that weapons that do not have dynamic frequency adjustment aparatus cannot change their frequency according to target, thus inflicting different amounts of damage depending on the target hull material.