The Xubuntu version numbering scheme is based on the date we release a version of the distribution. The version number comes from the year and month of the release rather than reflecting the actual version of the software.
The first official release of Xubuntu (Dapper Drake) was in June 2006, so its version was 6.06. This version (Lucid Lynx) was released in April 2010 so its version number is 10.04.