This last of Baum's Oz books is one of his very best, but unfortunately it seems to contain more internal contradictions than any of his other books. This may be due to the circumstances of the book's composition. As I understand it, Baum wrote both "Magic" and "Glinda" a few years before he died and put the manuscripts in a safe so that, if something happened to him (his health was already poor), they could be published to support his wife. He then wrote "Tin Woodman" and submitted it to the publisher, but after that instead of writing a new Oz book he took "Magic" and "Glinda" out of the safe and revised them. I have noticed several contradictions in "Glinda" that may be due to this manner of composition and to the fact that Baum finished work on it shortly before he died and therefore did not, I believe, have a chance to check the proofs.
First of all, Glinda, Ozma and Dorothy have a long conversation about the reason Ozma and Dorothy are going to travel to the Flatheads' mountain and the Skeezers' lake, and the best way to deal with the situation. Yet a few pages later, Baum says that although Glinda was the only one who knew that Ozma and Dorothy were going there, even she didn't know what their errand there was. Perhaps this passage was carried over from an earlier version of the book which had a different opening.
Later, when Glinda finds out that Ozma and Dorothy are trapped on a submerged, domed island, she makes a small island, covers it with a dome and sinks it in a pond near her castle; then she tries unsuccessfully to bring it to the surface. At a later point, however, Baum says that Glinda's magic could have raised any "ordinary" sunken island.
Another possible inconsistency arises from the fact that when Ozma's friends first meet in the Emerald City to discuss how she and Dorothy might be rescued Scraps suggests draining the water out of the lake, and no one pays attention to her. Yet when she later makes the same suggestion once the rescuers have arrived at the lake, the suggestion gives Glinda, the Wizard and the Adepts an idea of how they may actually reach the island. Then again, Scraps' making the suggestion the first time may be an example of forshadowing on Baum's part, rather than an inconsistency.
Finally, the chapter which concludes with Ozma and Dorothy emerging on the top of the Flatheads' mountain and seeing the Flatheads for the first time has the most undramatic, unexciting ending of any chapter in any Baum Oz book. If "Glinda" were a television program rather than a book, it would be appropriate to cut to a commercial after a shot revealing what the Flatheads look like; but no matter how strange their appearance is, a long, detailed verbal description of the Flatheads is not an even slightly gripping way to end a chapter. Perhaps, had Baum lived to read the proofs of the book, he would have improved this chapter ending, or perhaps the people who put the book together from Baum's manuscript were mistaken about the place where he intended the chapter to end.
On an additional note, this book has an interesting foreword--a note from the publishers, explaining to young readers that Baum has passed away. In it they claim that Baum left some unfinished notes about Oz. It is unclear how much truth was in this statement. "The Baum Bugle" once published a fragment supposedly by Baum consisting of the opening of the first chapter of an Oz book that was never continued or given a title, although I believe some scholars have suspected this text was the work of Baum's son, Frank Jr., who wrote an Oz book called "The Laughing Dragon of Oz" during Thompson's time as Royal Historian that precipitated a legal battle between him and Reilly & Lee. However, I once saw quoted (possibly in "The Annotated Wizard of Oz") a letter Baum wrote to his publishers saying that he had completed the Oz books for 1919 ("Magic") and 1920 ("Glinda"), and that material existed for the 1921 book. In any case, the publishers' statement in their foreword to "Glinda" that they would fit together Baum's notes and give readers more stories of Oz turned out to be preparation for the deception they practiced the following year, when Ruth Plumly Thompson's long, successful reign as Royal Historian of Oz began with the publication of her first Oz book, one of her finest, which Reilly & Lee falsely claimed was written by Baum and only edited and completed by Thompson: "The Royal Book of Oz."

I think Ozma is having delusions of grandeur in this book. She claims to be a powerful fairy who came down from her sister fairies to rule the land of Oz. Well, powerful, yes, with the help of some handy magical tools. And "fairy"...? Ozma, according to book 2, was the daughter of the ruling king, the perfectly ordinary human heir to the throne, brought up in relative poverty as a boy under the care of an unscrupulous witch!

The 37th book in the Oz series, "The Magical Mimics in Oz" by Jack Snow, says something about the Fairy Queen having left the baby Ozma with King Pastoria to be raised by him and be his heir--in other words, she was a fairy and was not Pastoria's biological daughter. Presumably Snow came up with this as an explanation for the inconsistency you pointed out, Shira.

In my first posting on this board I forgot to mention one of the oddest features of the book's plot, which I suspect is also due to Baum's dying before he could check the proofs. After the Skeezers' domed island has been submerged in the lake, a chapter ends with Dorothy deciding to continue looking out through the dome and searching for the three Adepts, who are enchanted in the form of fish. There is no suggestion that either Dorothy or Ozma has yet decided the situation is serious enough for Dorothy to use the magic ring Glinda gave her to make the Great Alarm Bell at Glinda's palace sound. Yet at the beginning of the next chapter the Great Alarm Bell sounds, leading Glinda to ascertain what Ozma and Dorothy's situation is through her magic. At no later point in the book does Baum ever explain precisely how Dorothy came to the decision to use the ring. It wasn't actually necessary for him to include a scene in which she decides to do so, but at the very least he should have had Dorothy or Ozma explain to Glinda once they are reunited how they decided that Dorothy should use the ring.