Why would an iOS update fail when normal iOS sync-type operations continue to work just fine? And why does using a USB 3 port for those iOS updates work without problem?

I recently had to update my iPad and iPhone to iOS 12.4. No big deal,I've done those updates lots of times already. I have an Apple cable plugged into my Dell monitor, a USB 2 port. I've used that port for years, no problems updating the iOS. However, for both iPhone and iPad, the 12.4 updates failed. With my iPhone, I spent about 30 minutes on the phone with Apple support. What worked was using a different Apple cable plugged into a USB 3 port.

After this issue with my iPhone, I have continued to do normal sync-type operations with both devices to update Calendar, Contacts, move photos, etc. No issues there. But when the same issue happened last night with my iPad, I moved that original Apple cable to a USB 3 port. No problems doing the iOS update.

So I determined that the issue was the USB 2 port, not the Apple cable. The USB 2 port is in my Dell U24xx monitor, and Windows sees that monitor as a generic uPnP display. The monitor has a USB hub, and I have a USB cable connecting the monitor and a USB 2 port on the back of my motherboard.