GMS v139 - Demon Avenger

The only thing that reduces the damage lost from physical resistance is elemental resistance. DBs do not have this.

I've been told the same but given no explanation; how exactly does elemental resistance cause physical resist to affect your damage less?
Is it like having say Ice elemental skills would cause you to do less damage against LHC mobs but up against something like Hilla since you have both ice and phys attributes to your skill it deals more damage than if it was one or the other alone?