Why did my Nexus 4 download the Jelly Bean 4.3 update again?

Why did my Nexus 4 download the Jelly Bean 4.3 update again?

I've got a stock, non-rooted Nexus 4 which I updated to Jelly Bean 4.3
when the OTA update came out a month or more ago. Just today I got another
message that there was a system update available. It said it would update
to 4.3, and I double-checked (and took a screenshot), and confirmed I
already had it in "About phone."
Nonetheless, I went ahead with the update to get rid of the notification,
and it went through the usual process. 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. I can't find any
differences after the update. Does anyone know why this might have
happened?
Before screenshot, taken 2013-08-23 16:28:04, before the update was
started about a minute later:

After screenshot, taken 2013-08-23 16:36:14 after the update finished and
it restarted.