Wicked the musical? Does anyone know why the last note in "I'm not that girl" is so low?

I have seen Wicked 3 times and watched several Youtube videos and I think even the best singers, "Stephanie J. Block, Kerry Ellis, etc." sound awkward singing that bottom note. I am no musical theory expert, but is it something to do with the key that it is sung in or the major/minor chord??? Just wondering if anyone knows why Stephen S. decided to write it that way?