Thanks, Jim. I suspected that this might be the case, and I should have looked this up in the appropriate place.
It's possible that in another edition, someone may want to add sound files to this EPUB, and so between that and the desire to use the most current format possible, I exported to EPUB3. However, if I simply change the version number to 3.0 from 2.0, I'm not sure what effect that will have on the existing tags and structure. I really don't want to go back through this book and make more changes at this point, when everything is behaving well.
I can make the correction to the dcterms:modified property and then change the version number to see if that affects anything. But I'll certainly plan on exporting to EPUB2, then, in the future for EPUBs that are not multimedia-rich.