I don't know if this is a bug, but it doesn't hurt performance. It may just be cosmetic.
See attached image.
When you fool around with priorities (per file) the progress bar wants to go to some number around 35%. When you return all files to normal priority the progress bar goes back to the correct number.
Content | Progrss Bar
Re: Content | Progrss Bar
All priorities, or specifically "Do not download?" It may be adjusting the completeness percentage based on which files you have or have not selected...?