Why it fails to specify the permission for backup files with option --chmod when using rsync command?

Issue

When trying to backup files with rsync command as follow:

# rsync -v -rltD --chmod=u+rwx,g+rwx,o+rx /var/log/ /tmp/log

The permission of backup files failed to specify to 775 but became to 755.

# ll /tmp/log/
total 13876
-rwxr-xr-x 1 root root       0 Oct 31 16:35 aaa
-rwxr-xr-x 1 root root    4627 Apr 21  2014 anaconda.ifcfg.log
-rwxr-xr-x 1 root root   32542 Apr 21  2014 anaconda.log
-rwxr-xr-x 1 root root   53057 Apr 21  2014 anaconda.program.log
-rwxr-xr-x 1 root root  152244 Apr 21  2014 anaconda.storage.log