Perhaps it’s got something to do with this reply (you may need to see the thread for a better context):
The bug exists because old deleted posts are eventually removed from the database, and I didn't account for this when implementing the selfmod stats, so the data on which posts were created by which users is missing for old deleted posts.
The message will now say something like "(9 posts by 1+ users with 3 merit deleted.)". The first and third numbers are accurate, but the second number is imprecise; the second number will actually be "distinct users whose posts were selfmod-deleted and whose deleted posts are not very old and therefore still happen to be in the database." I could add more logging so that an accurate count could still be determined, but I don't think that it's very important to give the absolute correct number here, especially since any recent selfmod activity will still increase the count.
I’m not really too acquainted with self-moderated threads, so I haven’t got enough background on how this looks like over time on a given thread. In fact,
I though Merits were still shown on the self-moderated threads, but there are none visible in the OPs thread on the said stats. (probably none involved)