We did some restructuring in the book between the third and fourth edition, and it's quite possible that we used something before we defined it as a result. That's the way book writing goes sometimes... you get three authors working on things, an editor who is supposed to check for details like that, and a bunch of reviewers who are also supposed to catch it, and little things like that still slip through the cracks.

Be sure to report this as a bug against the book. That way we can track and fix it for the next edition, if not the next printing.