Why did my Nexus 4 download the Jelly Bean 4.3 update again?
The before and after screenshots of the about screen confirm that I had the exact same Android, Baseband, and Kernel versions, as well as the same Build Number


Actually, they do not. The first screenshot shows a build number of JWR66V, and the second is JWR66Y. An official changelog has not been released, but T-Mobile's documentation indicates that the update contains "security" updates. You can also find some (unofficial) discussion of the actual code commits in this article. So although the version number was not incremented, it is a different build than you previously had.