Versions of a version history are usually incremented this way : 1.0, 1.1, 1.2, ...
When unarchiving a content, the next checkpoint will create a new branch in the version history. Which means that the new version is named 2.0 and its predecessor is the root version and not version 1.2.

I am not sure whether or not this is a bug, but I guess this is not expected and could perhaps show to have some undesirable side effects.

More details are availables in the JCR specs :

• Repository Model $3.13.5 Version History
• Versioning