Misuse Of Word In Star Wars Book

I collected Star Wars toys and books for many years, starting in 1977 and stopping sometime after the release of Revenge Of The Sith.  There were several books I had never gotten around to reading, and the writers’ strike has now given me time to do so. Well, I just finished reading Star Wars X-wing Book Three: The Krytos Trap. I had difficulty enjoying it because the author, Michael A. Stackpole, misused a word in the book’s second sentence. Here is what he wrote: “Rogue Squadron had come to mourn the passing of one of its own on the week anniversary of his death.” I read the sentence many times, trying to make sense of it. I figured it had been a year since the death of the character, and Stackpole meant to write “the anniversary week.” That’s what I thought for quite a while until it became clear many pages later that only a week had passed. And then it became clear that Michael A. Stackpole has no idea what the word “anniversary” means. It means, “the yearly return of the date of some event.”  There is no such thing as a week anniversary or a six-month anniversary. The word comes from the Latin annus, meaning year. From that word, we also get annual, which means yearly. The writer should be thoroughly embarrassed for making such a mistake. And why did the editor not catch it?