"That's a big part to the ending of Wicked. The reason she was soo powerful and different was because she was 'a child of both worlds'."
BUT if you notice, Nessa and Elphaba (in the novel) are different physically- Nessa being armless and obviously Elphaba's skin. The normal child, their brother Shell, is the child Melena has with her HUSBAND.
And the novel, Elphaba isn't that strong at magic.
"During this performance, please feel free to let your cell phones and pagers ring willy-nilly. However, do remember that there are heavily-armed knights on stage and you might well be dragged up and impaled."
(Pre-curtain announcement at the new Broadway musical Monty Python's Spamalot)