The Ubuntu 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.

Our first release (Warty Warthog) was in October 2004 so its version was 4.10. This version (Jaunty Jackalope) was released in April 2009, so its version number is 9.04.