The Kobo driver does not update the metadata on the device except for the series information. This is a deliberate design choice, but might change in the future. If you want to update the metadata, you can use my Kobo Utilities plugin to do this, plus a lot of other things.
But, as GeoffR mentioned, the downloaded books can have subtitles. To empty this, you need to run the SQL statement that he posted. I don't know why I don't have the utilities doing this.
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