yeah, something like this is what's most plausible to me; my first thought was 'the editor completely forgot to ask for this until five minutes before it needed to go to print', because I've done that sort of thing myself. I do not think it is a cost-saving measure or anything like that.

Had a vaguely related experience: 2nd book (narrative history) came out without chapter-by-chapter source notes or index because the publisher went to print 3 months earlier than planned to confect a clash with a senior author publishing at the same time! Fortunately they were added for the reprint.