Question about Xibaqua and maximum hit taken

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% with 80% Lightning resistance and 50% with 80% chaos resistance.

Did I understand anything wrong? Or is this a real improvement on my defenses?

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This is the pob: https://pastebin.com/iN0tFvgE and you can activate xibaqua yourself and see it