gsutil: List only directories (bug?)

$ gsutil ls -d 'gs://XXX/test/**/' 
gs://XXX/test/
gs://XXX/test/336x280.swf.gz

So it lists not only directories (ending with /) but also regular files (ending with .gz for instance).

But accordingly my understanding this command should list only directories. Where is the error?

$ gsutil --version
gsutil version: 4.27

Or is it a gsutil bug?