[Adoby]

To update the actual contents of a ebook is expensive. It requires the computer to make a copy of the book, unzip it, change the metadata, zip it and finally replace the original copy.

When you are fixing metadata, perhaps in bulk, the metadata may be changed several times. It would slow down Calibre a lot if the actual books had to be changed every time the metadata is updated. Instead most of the changes are only stored in a separate database file that is much quicker and easier to change.

This is why the metadata inside the book only is updated when the book is converted, saved to disc or sent to device.

It is possible to manually request that Calibre update the metadata inside ePub books, using the plugin "Modify ePub".