[Derision]

There is code in Client:: Damage which cuts all PVP damage to 2/3 of normal, so that is probably causing it.

Simple fix would probably be to change the code in question to this:

//Don't do PvP mitigation if the caster is damaging himself
if(other && other->IsClient() && (other != this) && damage > 0) {
                int PvPMitigation = 100;
                if(attack_skill == ARCHERY)
                        PvPMitigation = 80;
                else
                        PvPMitigation = 67;
                damage = (damage * PvPMitigation) / 100;
        }