It probably ought not be evolution, unless Elspeth is actually in a position to give an account of human evolution that is consistent with what she knows about vampirism, lycanthropy, or witchcraft. In which case I would totally love to hear it, because I can't think of one.

Her power does vary in effectiveness depending on how closely she has verified the thing she says (e.g. if at age four she said "My daddy is dead", this would ring true, but not as true as "I'm four", because she believes the first thing secondhand and knows the second thing firsthand). And it's not actually impossible to doubt her even at her maximum truthiness. She conveys that she isn't lying (and isn't a hallucination or otherwise basically untrustworthy), but she doesn't come off as an Omega-creature who is absolutely beyond the possibility of being mistaken.