Why the Change, Lewis?

I'm posting on a question I had once I read the actual myth in the back of Lewis' Till We Have Faces: A Myth Retold. In the true myth, according to Lewis' own synopsis, Psyche had both sisters come to her after she married Cupid. But that's not the difference that really makes me wonder. I wonder why Lewis changed the method of convincing Psyche to use the lamp that night. In the myth, the two sisters are overwhelmed with their jealousy and they convince Psyche to use the lamp because they knew it would destroy her happiness. In Lewis' version, Orual seems to have selfish motives (she wants Psyche back with her, to agree with her, to need her), but she's also seemingly convinced that her intentions are for Psyche's benefit. So I guess my question is two-fold.
One: do you think the accounts are different, or is Orual actually acting to destroy Psyche's happiness for her own sake, and...
Two: if they are different, why do you think Lewis changed it? Or even if you think they're the same, why did Lewis make his account so much more complex? I think even if you argue that Orual really understands what's going on, but she's deceiving herself to justify what she's doing, that's still far more complicated than the original myth that just states how the sisters behave selfishly.
I can't decide how I feel about the first question, whether or not Lewis is changing the motive or not, because I keep going back and forth on whether Orual really does know what's happening but she's just deceiving herself to justify acting in her own best interest. However, I think regardless of the answer to that question, Lewis is definitely making the situation more complicated. And I think one reason may be so that Orual can be more relateable, more understandable, more sympathetic to our own experiences and feelings. We can easily put ourselves in that situation and feel justified making either decision. Whereas, in a situation where we only act out of selfish intentions, it would be much harder, if not impossible, to justify ourselves acting that way.
This is getting long, and I think I've made my questions clear, so I'll leave you at that. Let me know what you all think!