Ubuntu Security :: Clean Mask For Files And Folders Through Samba

Still working on the mask of files for shared folder. I now have a shared folder with the exact behavior I expect :
Code:
sudo addgroup share_group
sudo mkdir /media/volume/shared_dir
sudo chgrp share_group /media/volume/shared_dir
sudo chmod g+s /media/volume/shared_dir
sudo chmod 770 /media/volume/shared_dir
sudo setfacl -d -m group::rwx /media/volume/shared_dir
sudo setfacl -d -m other::--- /media/volume/shared_dir
emma@box:/media/volume/shared_dir$ ls -al
total 8
drwxrws---+ 2 root share_group 4096 2010-02-09 12:53 .
drwxr-xr-x 8 root root 4096 2010-02-09 11:58 ..
-rw-rw----+ 1 emma share_group 0 2010-02-09 12:53 test
By default, user from the group can modify this file. That's perfect.

I have define the share in Samba this way :
Code:
[share]
comment = Shared Folder
path = /media/volume/shared_dir
browseable = yes
guest ok = no
read only = no
hide dot file = yes
# force group = share_group
# create mask = 0660
# directory mask = 0770
# force create mask = 0660
# force directory mask = 0770

When drag & dropping a file in this share, here is the default mask:
Code:
emma@box:/media/volume/shared_dir$ ls -al
total 192
drwxrws---+ 2 root share_group 4096 2010-02-09 12:54 .
drwxr-xr-x 8 root root 4096 2010-02-09 11:58 ..
-rw-rwx---+ 1 emma share_group 6148 2010-02-09 12:54 .DS_Store
-rw-rwxr--+ 1 emma share_group 176684 2009-12-21 23:33 IMG_7487.jpg

So the dropped file have execution rights for the group, and read access for other. I expected it to have the same rights than the file created directly using the touch command. I tried to play with the mask options, without success. The file has been dropped from my mac, which is a Unix like OS. I guess that some authorization access are inherited from the original file, for the user and other parts. But where does the group authorization come from ? Moreover, is is possible to define in samba a default mask, whatever the authorization of the original file?