chmod: permissions of a directory don't change

Question

I have some issue with chmod:

    mis@hell:/var/www/blah$ sudo chmod g+w .git -v
    mode of `.git' changed from 0755 (rwxr-xr-x) to 0775 (rwxrwxr-x)
    mis@hell:/var/www/blah$ ls -la | grep git$
    drwxr-xr-x 1 www-data www-data   442 Mar  6 09:30 .git

It looks like chmod doesn't change permissions of a directory in fact (rwx vs r-x for group). Why?