An update can consist of 4 parts:
system image
boot image
recovery image
radio image
Google builds the system image, boot image, recovery image from their source code.
Samsung and Verizon build the radio image.
In the system image, there are a handful of files that need changed because the Nexus is on a CDMA network. Google does not have the required licenses to re-distribute these files, they must come from Samsung and Verizon.
Google has a penta-band unlocked reference device. They build a version of Android for it, and test on it. When it passes their testing, the update is ready as far as Google is concerned. Verizon and Samsung then take that update (probably in the form of source code, not zip files) and make the required changes to allow the Nexus to work on a CDMA network. When finished, they give the green light for the update to be distributed from Google's servers.
Take everything above, and apply it to LTE as well, since it's a very closed and proprietary standard.
Because these versions are different when built, they need a different version number. This is why the GSM Nexus and CDMA Nexus will never be on the same version.
4.0.4 is the current version of Android. Any fixes special or specific to the CDMA Nexus are not included -- because Google does not have license to include them.
4.0.5 (maybe) will be the version for CDMA. It will never be in AOSP, because it uses code that is not open source.
Now is the time to ask any other questions. I'll try to answer if I know the answer.