[Manichean]

If the Calibre reader reads the faulty metadata, my personal guess would be that the Mobipocket Reader hasn't updated the metadata, but instead has written the correct information into some database of its own (like Calibre would do, as well). In that case, there's no way for Calibre to know the correct metadata.
Or, to say it in a more direct way: Calibre seems to read what's inside the files. You need to find out how to change what's inside, though I'd suggest just doing it in Calibre.