The story of Oz has been retold countless times, so it's fitting that storytelling itself is a major theme in this book. Wicked is all about re-telling the story of the Wicked Witch of the West. But there is never just one version of any story in the book, and this is particularly true of the Witch's. Her story is just one of many in Oz that gets retold and reinterpreted over and over again.
Arguably, Elphaba's life story (which is actually made up of several different stories) is separate from the story of the Wicked Witch. Wicked complicates this theme even further (which it has a tendency to do) by questioning the truthfulness of communication and storytelling itself. There are multiple versions of stories floating around, and every story contains a combination of truth and lies. Telling and hearing stories ultimately boils down to interpretation and perspective. For a book that's a "re-imagining" of another narrative, it's fitting that communication and stories are not static or fixed.