GID behavior

[Paul Lew <paullew@cisco.com>]

According to zsh manaul:

     GID <S>
          The real group ID of the shell process.   If  you  have
          sufficient  privileges,  you may change the group ID of
          the shell process by assigning to this parameter.  Also
          (assuming  sufficient privileges), you may start a sin-
          gle command under a different group  ID  by  `(GID=gid;
          command)'

This is what happened:

$ id
uid=46991(paullew) gid=25(eng) groups=25(eng),3010(cdseng),3011(cnsar)

$ GID=3011 touch x
$ ls -l x
-rw-rw-r--   1 paullew  eng             0 Jan  8 23:48 x

$ chgrp cnsar x
$ ls -l x
-rw-rw-r--   1 paullew  cnsar           0 Jan  8 23:48 x

$ GID=3011

$ echo $GID
25

$ echo $ZSH_VERSION
3.1.9-dev-7

$ uname -a
SunOS cns-build2 5.6 Generic_105181-16 sun4u sparc

So, bug, feature, or I missed something?  Thanks in advance.