Comment 4 for bug 893842

This appears to be deliberate. Here's Steve's changelog:

    - Don't set up the admin group or add the user to it; the sudo package
      now always sets up the sudo group on install, so we can as well use
      this group as the admin group. This will be inconsistent with
      previous Ubuntu releases, but consistent with Debian and compatible
      with sudo.

I think, for compatibility, policykit-1's configuration should allow both the admin and sudo groups.