In my opinion the explanation is easy: it comes all down to conditional probability and Bayes' theorem:

Conditional probability and Bayes' theorem tell you that how given some "ground belief" and new facts, the ground belief should be adjusted to incorporate the new evidence. Making this part of your daily life and belief system is what rationalism is about.

But what happens if your ground truth is "fucked up" (in the sense of how an average person would see it)? Then it can easily happen that new evidence can perfectly explained by your ground truth/belief system and thus (in a very rational sense) actually strengthen it.

Also keep in mind that a lot of things in the world are "messy", so it's not so hard to come up with a belief system that gives an "encompassing" framework that actually "explains" more things. If this system than becomes "strengthened" by incorporating lots of additional seen evidence (again using conditional probability and Bayes' theorem), this leads to a similar situation.