By default, Mac OS X treats all directories as if they have the Set-GID bit on.  When a directory or a file is created, the group permissions will be set to the same value as the Parent and not the user default GUID.

• Here is an extract from the Apple OS X open(2) manpage

     When a new file is created, it is given the group of the directory which contains it

• Example:

$ id

uid=501(admin) gid=20(staff)   groups=20(staff),12(everyone),204(_developer)

$ mkdir testdir 

$chgrp 204 testdir/

$ ls -lsd testdir/

0 drwxr-xr-x 2 admin _developer 68 Sep 26 11:16 testdir/

$ touch testdir/file1

$ ls -led testdir/file1

-rw-r--r-- 1 admin _developer 0 Sep 26 11:18 testdir/file1