Well, I am no math genius but I am rather sure that I do better math than the progress bar on old lists. I have e.g. this list: "HSK 1 (old Hànyǔ Shuǐpíng Kǎoshì) (Remix)". I have studied it before and recently nuked my account. Now I study that list again and I am around 22% done with the list but the progress bar do only report 0,1%. I did restart the list from section 1 and finished one more section but the progress is still only at 0,1%. I have restarted it again and added more words and is now up to 1,1%. Seems that the progress bar only calculates from where skritter started to add new words from the list. If I add new words now it do increase but do not count the old words. Similar things happens with other lists but not all of them. Can't really figure out what this is about. It is not important though, mostly just amusing.

Ah a bug! I've fixed it, the math should be much closer to reality in the near future once the fix is uploaded. Thanks for the report.