Why might the App Store draw variable-type strings in place of actual values?
This issue resolved itself after that single update. 
I did reboot once without updating, and the glitch persisted past that boot, but after the update in question was done and the laptop rebooted, there was no recurrence.

As for the likelihood of a corrupted installer dmg, @KevinChen points out that "there is a lot of checksumming that goes on when you open DMGs and apps, so corruption seems unlikely"