git-checkout changes the index but not the checked out files?

I am a bit confused by git-checkout behaviour in the following scenario.

$ git version
git version 1.5.1.gf8ce1

$ git-branch
  mdl17-polhn
* mdl18-local
  mdl18-topnz

$ git-status
<shows a clean status>

# checkout an upstream branch to test that it has a bug
$ git-checkout origin/MOODLE_18_STABLE
$ git-status
<shows a clean status>

# switch back to our local branch to test that the bug is gone
$ git-checkout mdl18-local
$ git-status
<shows a very dirty status - the index has changed but the files
weren't updated to the contents of mdl18-local>
$ git-reset --hard # this fixes things back to normal

Is the bug in git-checkout or in my understanding? ;-) Perhaps I am
modelling my use of git-checkout on how I used cg-switch and that's
wrong. But reading the man page, it says:

When <paths> are not given, this command switches branches by
updating the index and working tree to reflect the specified
branch, <branch>, and updating HEAD to be <branch>

cheers,


martin