I didn't realize that Montessori is considered a generic term and not trademarked [1]. Since people buy apps before they try them, the name is probably a big driver of the app's success.
Similarly, the guy who generated 800,000 books uses "Websters" in the book titles for his best selling books, which is also not trademarked [2] and can be used freely.
[1] American Montessori Society, Inc. v. Association Montessori Internationale, 155 U.S.P.Q. 591, 592 (1967), referenced in the Montessori Education Wikipedia article.