Just to add a bit more to Burton's comments:
The reason for the disparity between the release of an Android version and it showing up on the Nexus phones is due to the customizations needed. [Yes I know it's stock Andorid, but there are still some custom tweaks done plus additional drivers and software added. See my last paragraph]. While I'd imagine Android is developed and tested on the Nexus phones; what they seem to do is release the "base" Android and get that to 100%.
After it's released, they go back and update, test, and re-certify the apps that are on the Nexus phones.
Think about it: if you look at the Emulator in the Android development tools, it's very basic; it doesn't even have the "cube" scroll list in the apps drawer. So I think the process is, release the base OS to the public domain, then re-test, modify, etc the base OS to work on the Nexus production phones.
The Nexus S was released when Android 2.3 was released, and so seems it didn't get the full vetting it normally would. While I wouldn't call it "buggy" (I have one, BTW), it does have a few bugs in there that Google probably doesn't want to have show up on the N1.