In the other it temporarily inconveniences the disarmee - he has to scramble to get either his own weapon or another one conveniently left around. Mechanically, what does a temporary[/] disarm effect? The target can't use his "Weapon" keyword powers to full effect, because his primary weapon isn't available. See the conditions table for ways to mechanically represent this. So you have powers that, when you are fighting a weapon-wielder, represent the weapon having been disarmed until they save, or the power ends, and they can recover it; but that power isn't gimped against a non-weapon user.