Mac creating files w/ wrong perms on samba share

In my group, which is very heterogeneous in terms of machines, we use a samba share to collaborate on files and such. In all but one case, it works as expected (or at least close enough).

The one exception is my boss' laptop, a snow leopard macbook air. On his desktop (also snow leopard), if he creates a file it ends up serverside with perms of 774, but when he creates it with the Air, the perms are 644. The key problem is the lack of group write permission on the laptop created files. What's really confusing is that everything that I've looked at on the two machines are identical - same version of OS X, same version of samba (3.0.25b-apple), same settings for the same software, etc. I can't imagine why one machine would be different than the other, but it is.

To try to be complete w/ the description, here is the relevant portion of my smb.conf file:

comment = my Share
path = /path/to/share
public = no
writeable = yes
printable = no
force group = myshare
directory mask = 0770
create mask = 0770
force create mode = 0770
force directory mode = 0770

EDIT: I looked at three more Macs and all of them worked as expected which leaves this one laptop the true oddball. This wasn't as good as a test as the others though, as they were all leopard.

try setting

unix extensions = off

in your globals, more details at apple's forums