The list of files in the Desktop app doesn't match the list in the Dropbox folder or the list on the dropbox.com site. For example, there is a Mac folder in the Desktop app and on dropbox.com, but not in the Dropbox folder.
I've looked more carefully and find I was somewhat mistaken. The file lists of Desktop app, website, and iPhone do match. The file list of the, Dropbox folder, which I assume is part of the Desktop app, lacks the "Mac" files that the other three have.
Initially, I emptied the Dropbox folder and reinstalled the Desktop app. After synchronizing everything, the Dropbox folder lacked several files that the other places had. I had to drag other files to the folder to use them.
As an aside, I'm quite impressed with Dropbox. I read every document and watched everything on the website. Of the two friends to whom I've enthusiastically explained the functioning, one immediately started using it, and the other is leaning that way.