A similar thing happened to me with a 6300 that I used to own. I updated to v6.0, then later NSU said v5.5 was the latest. It turned out that they had pulled the v6.0 from some product codes due to an issue.
If you bought the phone second hand, it could also be the case that the previous owner has hacked the phone using unauthorised software in order to update to an unavailable version.