I think it'd be funny if Shiva doesn't even remember her own origin story. Like the one she's giving to Cass in that journal is what she thinks would be best for Cass to hear, to give her closure. But in her mind, she woke up at 19 as Lady Shiva and there was nothing to it before that. Like maybe all the origins she told were supposed to be ones that suited the people she told them to but then she stopped giving a shit and started handing out a new origin any chance she can. She's just as mislead by her stories as everyone else.
wouldn't that be a blast