I just counted and, including the TOC etc there are 166 double-sided pages, so 332 pages total. If I had to guess, the original plan was to have the Batman book include the full issues (including the backups) like with Death of the Family and the Court of Owls books, with the Joker HC containing the tie-ins and the backups again, and maybe an issue or two from the main series. At some point that changed and they didn't update any of their solicitation text, which is unfortunately pretty common at this point.