Why does Drupal say I need to upgrade to 8.0.0 beta 6 when I'm already checked out on that tag? I followed the git instructions at: https://www.drupal.org/project/drupal/git-instructions to get Drupal 8 :

git clone --branch 8.0.x http://git.drupal.org/project/drupal.git .


I then checked out the tag for 8.0.0 beta 6:

git checkout tags/8.0.0-beta6


And this command confirms I'm on that tag:

$ git name-rev --tags --name-only $(git rev-parse HEAD)
8.0.0-beta6


I then prepared for the install: set up a blank database and user and then used this drush command to do the install:

drush site-install standard --account-name=admin --account-pass=admin --db-url=mysql://d8:d8@localhost/d8


When I go into the admin -  admin/reports/status - I'm told that I'm using 8.0.0-dev and that Unknown release date (version 8.0.0-beta6 available) is available.

But why? when I was already on 8.0.0-beta6 branch before I ran the install.
update.module doesn't know about git.

It only understands .info(.yml in D8) files. And .info.yml files only get their version as part of the package building process. If you look at your system.info.yml, it doesn't say anywhere that it is beta6.

There is a module called git deploy, that is trying to fill that gap and extracts version information from git and tells update.module about it, there is a Drupal 8 release but I don't know if it works with the current version.