Just a guess, of course, but I would think that the update does not correctly recognize the OS signature because at the time it was unknown. these things get updated continually, so maybe wait 2 or 3 weeks before a new install package is available and fixes the problem or a knowledge base article offers a workaround...
On my dual g5, I'm up to 13.0.2. You might go looking for a new update .dmg?
The Auto Updater has kicked in twice and failed to install the update 13.0.1, and I have downloaded the DMG directly from Adobe, which has also failed to update my copy of Illustrator. I'm not sure where else to get the update. I do know that the update was posted on the Adobe site in July 2007 and my OS version updated in August 2009.
It seems strange as well that the Adobe Updater took nearly 2 years to advise me that an update is available.