git diff and file deleted from the cache

The manual of git-diff tell me that :
  git-diff [--options] <commit> [--] [<path>...]
      This form is to view the changes you have in your working tree
      relative to the named <commit>. You can use HEAD to compare it with
      the latest commit, or a branch name to compare with the tip of a
      different branch.

So the following seem strange :

$ echo foo > bar
$ git commit -m "committing bar" -a
Created commit 074893b: committing bar
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 bar
$ git rm --cached bar
rm 'bar'
$ git diff HEAD --

diff --git a/bar b/bar
deleted file mode 100644
index 257cc56..0000000
--- a/bar
+++ /dev/null

@@ -1 +0,0 @@
-foo

I excepted the diff to be empty, as HEAD and the working directory are
synchronized, I've only modified the cache.

-- 
Rémi Vanicat