Well, for one thing, even fangraphs admits quantifiably measuring catcher defense is difficult at best.
And they don't incorporate framing into their catching defense measurements.
https://library.fangraphs.com/defense/catcher-defense/
So I choose to believe my eyes, and my experience...if you're routinely giving away free bases, you're not playing good catcher defense, even if you're gaining a strike or 2 per game (which I don't concede, BTW.)
Or, trust fangraphs' catcher defense rankings. They might be correct.
But it's not proven science. It's theoretical.