When editors get passes of deigned files, they don't have access to the indesign program. They get it as a pdf. It's really not the same thing.

Once a book gets transmitted to the design department, the programs they use are no longer Microsoft Word with spell check. So, any number of errors can get introduced when changes are requested after the book hits lay out and they become much harder to catch. Add in crashed schedules, overworked editors, designers, and managing editors…. And mistakes happen!