This is the first comment in this discussion I've seen that is actually interesting and might provide an explanation.

The Dropbox file status symbols are a -big hack-. Up until OS X 10.10, for instance, the file browsing interface didn't provide per-file badging at the OS API level. So they literally had to patch the running Finder process to do this.

I wouldn't be surprised to learn something similar is going on with Windows Explorer.