title_body: difference between Calibre's metadata.opf and content.opf
upvoted_answer: Calibre's metadata.opf is only used for Calibre purposes. If you change metadata entries with Calibre, the epub isn't immediately updated. IMHO, this is a major bug, but Kovid Goyal apparently considers it a feature. 
  
  If you want to apply the changes to the book, you'll have to select Save to Disk and save the book to a new folder. 
  Alternatively, install the Modify Epub Calibre plugin and select Metadata > Update Metadata.