confused by git diff --exit-code

With merge conflicts in the work-tree, diff's exit-code seems inconsistent. I 
thought --quiet implied --exit-code


/others/foo$ git diff --quiet
/others/foo$ echo $?
1

/others/foo$ git diff --exit-code
diff --cc foo.txt
index f3dc283,bea67fd..0000000

--- a/foo.txt
+++ b/foo.txt

/others/foo$ echo $?
0

/others/foo$ git diff --cc --quiet
/others/foo$ echo $?
0

/others/foo$ git diff --cc --exit-code
diff --cc foo.txt
index f3dc283,bea67fd..0000000

--- a/foo.txt
+++ b/foo.txt

/others/foo$ echo $?
0