Well spotted. I've noticed it too Peter. Sometimes someone who has made zero posts has a credit of one post. Must be an oddity of the plug-in that our tech admin installed when we tried to have a system where we could calculate the monthly posts of members. In the end we could not use it as those who were making posts on their mobile phones found that their posts were not being added to the monthly total. Obviously the plug-in has some shortcomings.
Maybe one day you can help us set up a system where we can accurately count the daily, monthly and annual posts that members have made. MyBB doesn't have a default system for doing it for each member.