Why is Joomla demo site showing it is using 4.6.0?
For background: The demo, training, test, and many local-dev systems for Drupal+CiviCRM and WordPress+CiviCRM are based on a common set of scripts, and they all install clean copies of Civi from git (using branches or version-tags). The demo sites are automatically rebuilt every night using the scripts. This keeps the sites up-to-date.

For Joomla: Installing a good, clean system for Joomla+CiviCRM from git is very difficult -- I and others have tried to automate it a few times and always given up. (Somewhere in my email history, there's a note-to-self about how hard this is. Alas, the language is too strong for publication.)

IMHO, there are two ways one can proceed with automating Joomla installation:


Don't use git. Develop a different set of scripts based on zip files. (The nightlies might be good for this.)
Rewrite the installer


At any rate... Without an automated installer, the Joomla demo site grows out-of-date periodically.