Patrick Durusau suggests an even shorter explanation:
There has been some confusion over the jump from 0.2* versions of Hadoop to a release of Hadoop 1.0 at Apache.
You have not missed various 0.3* and later releases!
People familiar with open source projects know that most of the time versions do not carry any meta-information about the maturity of a project. But the higher you go into an enterprise hierarchy the more important software versions are considered. So sometimes it is a good idea to bump up the numbers to something that is perceived by everyone as a mature version. Adoption is sometimes also about perception.
Original title and link: Apache Hadoop Versions ( ©myNoSQL)