I think packages are held back during upgrading when they require installing NEW packages in addition to upgrading existing ones. Perhaps your friend running sarge has those packages installed so samba is upgraded. If you want to upgrade samba and it is held back just:
apt-get -t testing install samba
Or you can simulate it to see what happens before you actually upgrade it:
apt-get -s -t testing install samble
You will probably see that it asks for permission to install some NEW packages.