Please note that this is the same codebase starting from 1.5 beta 3. Moving forward, all releases will start with this new major version of 2. The 6.x-1.x-dev development snapshot will no longer be published.
Initially the version jumped from 1 to 1.5 since there were pretty substantial changes but not enough to declare a "2.0" branch. That was my thought before actually understanding how versions are expected to be numbered on drupal.org. This caused some problems with the packaging scripts and made the releases inconsistent.
The next release will be the next beta of 6.x-2-0-beta1 with a final 6.x-2-0 released shortly after.