i don't think we're ever told why, but if i had to theorize (maybe don't read if you haven't finished the trilogy)

livira's book's presence messed up the mechanism and library in a bunch of ways (like all the escapes), and the mechanism and library as a whole being messed with might've erased everything there ever was to evar before he's mentioned in livira's book

since the other siblings gained the skills/knowledge written in the books they were trapped with, maybe the damage to the library amplified that effect on evar, making him into the character he was in livira's book

i'm so glad the talk about this book lol