Although no official word has been given, one of the reasons for the change could be due to the increase of page numbers. The book, that covers CLAMP’s career during its 20 years of history and their impact in the American culture, was originally expected to have 144 pages. Now, it is being listed as having 336 pages (more than the double).
The price has been increased as well by extent, from US$ 19.99 to US$24.99.
Thanks to shidouhikaru15 for poking me about that ^^