Thanks, that fixed it.
I am writing my own software that creates the IPTC-Keywords metadata entry, and I didn't realize that each separate keyword needs it's own IPTC-Keywords entry. I thought I could pack all the names into a single metadata item, but that is wrong. When I ran Irfanview, I was able to look at the metadata it created and understand that it's possible (and necessary) to create multiple metadata items with the same key (the IPTC-Keywords entry).
Zenphoto handles this perfectly.