chmod g=o

[Juam]

bug or feature?

How to repruduce it:
$ umask 0
$ chmod 007  1.txt && ls -l 1.txt | cut -b-10
-------rwx
$ chmod g=o 1.txt && ls -l 1.txt| cut -b-10
----r-xrwx
$ chmod u=o 1.txt && ls -l 1.txt| cut -b-10
-rwxr-xrwx

What is wrong: the write bit of the group is not set in g=o 
at least is not documented.
 
--------------------
$ chmod --version
chmod (fileutils) 4.1
Written by David MacKenzie.
 
Copyright (C) 2001 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

running in linux with libc-2.2.3