5.1 update downloaded to my Nexus 7, incorrect version number
After a couple of days, I decided to go ahead and apply the update. It completed normally and the tablet is now running Android 5.1.1, build LMY47V, which is what's expected for the 5.1.1 update.

It looks like the only problem was that the version number was displayed incorrectly. I'm still curious about why that happened, and why I haven't been able to find any other reports of the same problem.

UPDATE 2015-05-26: I've received the update on my Nexus 4; it correctly shows the version number as 5.1.1. The update size is 18.1 MB.