samba - Jan 2008 - clearing and setting the archive bit causes permissions for the owner group to change

I have Samba 3.0.26a in an openSUSE 10.3 machine.
I have set up a share like this:

	path = /dati/paolo
	valid users = paolo
	read only = No
	acl map full control = No
	create mask = 0760
	directory mask = 0770
	nt acl support = No
	map archive = Yes

If I access the share from a Windows 2000 client (logged as the owner of 
/dati/paolo) and create a file, I found the permissions set according to 
the "create mask" (-rwxrw----).
Then I change the permissions with the command

chmod g-w ...

and the permissions become -rwxr-----.
Finally, I clear the archive bit from the Windows client and the 
permission become -rw-rw----.

Well, I didn't expect the write permission for the owner group to be 
set. Is there something I am missing?

By the way, if I change the configuration of the share to

	create mask = 0740

then clearing the archive bit from a Windows client doesn't set the 
write permission for the owner group.