I believe rhufus is close to the answer. The downloader is multi threaded.
So I believe the 100% is for the first thread to finish, while there may be one or two more threads still in the process.
While the Bar Graph might show 100% because the one thread has finished, the rest of the file completes in the background.
The team is aware of this and since it is a ***** issue, it will be addressed after all the major issues come first.