I managed 89% on 371 orders but untracked letters shows 0%. My guess is the missing 11% is due to untracked letters but I select standard postage in the listing.

Remember a lot of loyalty programs use things like these to show how you are going. Some are accurate but there also some that aren't and I usually don't pay much attention but the ebay one is supposed to be linked to listing visability which I gather if you are below a certain level it may not be good when it comes to listing ranking in "best match".