Because Ubuntu is usually released in April (4th month) and October (10th month).
The Version number also indicates the year of the release - therefore the first released version of Ubuntu was Ubuntu 4.10 (20th October 2004).
According to Wikipedia:
Ubuntu releases are made semiannually by Canonical Ltd, the developers of the Ubuntu operating system, using the year and month of the release as a version number. The first Ubuntu release, for example, was Ubuntu 4.10 and was released on 20 October 2004.
Consequently, version numbers for future versions are provisional; if the release is delayed until a different month (or even year) to that planned, the version number changes accordingly.