This does seem to be a long standing Unix*-flavor behavior difference.  

The open(2) man page on OS X (and presumably FreeBSD) reads:

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

The Linux open(2) page has:

       O_CREAT
          If the file does not exist it will be created.  The owner  
(user  ID)  of
          the  file is set to the effective user ID of the process.  The 
group own‐
          ership (group ID) is set either to the effective group ID of 
the  process
          or to the group ID of the parent directory (depending on file 
system type
          and mount options, and the mode of the parent directory,  see  
the  mount
          options bsdgroups and sysvgroups described in mount(8)).

The Open Group Base Specifications (Issue 6, 2004) for open(1) has this 
rationale:

"The POSIX.1-1990 standard required that the group ID of a newly created 
file be set to the group ID of its parent directory or to the effective 
group ID of the creating process. FIPS 151-2 required that 
implementations provide a way to have the group ID be set to the group 
ID of the containing directory, but did not prohibit implementations 
also supporting a way to set the group ID to the effective group ID of 
the creating process. Conforming applications should not assume which 
group ID will be used. If it matters, an application can use chown() to 
set the group ID after the file is created, or determine under what 
conditions the implementation will set the desired group ID."

So the portable solution is to change the test to not assume which group 
ID is used.