What I see in my FOO is both my uBPDmom and uNPDbro become too emotionally invested in their beliefs and personal preferences. I used to be a bit like that

but over time, and thanks to my education, learned to see the world in a more nuanced way.
For instance, my uNPDbro is constantly on social media taking rigid stances on any number of issues and getting offended when people disagree with him. When I try to calmly explain other perspectives, or even my own somewhat differing point-of-view, he'll flip out and accuse me of just being a "devil's advocate" or whatever.
Meanwhile, my non-PD sister hardly ever expresses strong opinions unless it's in her field of expertise or affects her family in a significant way.