It's either a hardware difference or Apple artificially restricting where it installs to increase hardware sales.

It would not be the first time - the first iMacs are capable of running up to MacOS X 10.4, provided they have enough memory. I have successfully ran 10.4 on a beige G3. The installer, however, checks the hardware against a list of "forbidden" (if I remember the term used in the .plist file) machine types.

Here is a similar story, regarding 10.5:

http://www.macosxhints.com/article.php?story=200710260837463...