I noticed that the count reads "201 to 204 of 204 comments". It may be that these last 3 or so comments are whispers, and that is why they are not showing. This is odd behaviour, though.
Whispers don't normally cause this. This is a bug caused by users editing a post into a whisper. (there may be other causes, I haven't looked very deep into this)
You can make Vanilla re-sync the post count by deleting and then undeleting any comment in the affected thread. You can keep Vanilla from going out of sync again by applying the patch here.
Comments
You can make Vanilla re-sync the post count by deleting and then undeleting any comment in the affected thread. You can keep Vanilla from going out of sync again by applying the patch here.