I guess that Apple released it only as an update specific to the new hardware, even though it will install on other computers. Perhaps there are some bugs - i.e. carbonLib or video drivers - in 9.2 that they want to fix before a widespread release? My guess is that 9.2.1 will be released in a couple of months as an update for everyone else running 9.1.
Not the smell, the oppertunity to have a job-decription that includes second-guessing Apple. Not in the competative sense, just in the curiosity and R&D tracking sense. It was always nice to be able to stop and smell the roses in depth.