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