[itimpi]

At the point you have edited the metadata in Calibre, the revised metadata has not (yet) been embedded into the mobi file. The metadata will get inserted if you export the file from Calibre using the Save to Disk functionality.

One good reason to do a mobi->mobi conversion is to either get a new cover embedded in the file, or to embed a 'jacket' for displaying metadata at the start of the book. Those two features require a conversion and not simply an Export.