Once the recent updater security fix (as of 7/12/02) is installed, the system install program refuses to install the 10.1.3 , 10.1.4 or 10.1.5 updates, reporting that the drive does not meet the requirement. As I had to reinstall my system from scratch this weekend for some reason, I was stuck at 10.1.2 and couldn't install any later updates, with mounting frustration for a few hours.
After examining the script inside the system updates, I found that this is a result of the security fix having created a "softwareupdate" program in /usr/sbin/. The system updates for 10.1.3 and greater detects its presence and then would refuse to proceed. A way around this limitation it is to rename softwareupdate momentarily to something else (say "lala") in the terminal program via:
% sudo mv /usr/sbin/softwareupdate /usr/sbin/lalaand then run the installer. Afterward, don't forget to switch it back, so that you can use 'softwareupdate' from the command line, as described in this hint.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020718075551508