Was talking with Rage last night, and we looked up Parthans in the Diablo Wiki for some clarification. The Wiki notes that the DR granted by Parthans is additive and can technically stack infinitely, and thus make you invulnerable, though of course this falls off as monster CC immunity kicks in. My theory is that this affix was a holdover from Loot 2.0 that was never updated to work multiplicatively.
This would explain the feeling when you’re playing WW and group a massive pack of fresh density and literally nothing can kill you. The same can be said for a massive Fire EQ pull.