PoE The Screaming Eagle, Slam builds

Hi, I'm toying around in pob to improve defense vs elemental damage but there is a thing I don't understand. Let's assume I have 80% all elemental resistance. And I can take a maximum hit of lightning damage with 50 000 (maximum hit taken in pob). When I activate Xibaqua in Pob and still have 80% all elemental resistance and 80% chaos resistance. Xibaqua let me take 50% of elemental damage as chaos damage. Why does this double my maximum hit taken to 100 000? In my understanding it shouldn't make a difference if I take 100% of the damage as lightning with 80% Lightning resistance or 50% wi