There's just so much Ritsuka's dealt with, she just doesn't think to try to explain all of it. But for what it's worth, she does sound pretty confident in what she does say.

"Do you know if anyone's explained stuff here to him yet? Like one of the people who's been here a while and knows a lot about it..."

And is actually capable of giving a reasonably coherent explanation. Ritsuka tries but it's definitely not her forte.

"Because once he knows about that it'll probably make more sense to him. And if your past life was from the same world, then that's probably why he seems familiar."