From the description, it looks like the wrong signature dropped into the sort order for that copy and the one that was missing should have been where the missing one is.
Usually it's like two copies involved of a given item, from your description, someone else got two with your missing pages and none of what you got doubles of.
Common print facility issue, but not so common that everyone sees it all the time. The only time I've run across it was about 20 years ago when I got a 150-page or so classical music CD catalog and it had about the same type of issue in a similar manner to what you describe. One quick call to the company that produced the catalog, and a fresh one without the problem was on its way to me.