The studies I read suggest oxalate and phylate inhibit absorption. For instance

https://pubmed.ncbi.nlm.nih.gov/12088515/

As a layperson, perhaps I misconstrued bioavailabilty and absorption because both come into effect.

I glad your calcium levels have risen btw, I would wonder if other effects like exercise or overall dietary sufficiency have changed. If for instance you ate less foods with chelating agents you'd alter iron absorption, perhaps your dietary shift removed confounding phylate? And no matter why, it's great they rose.