@dfarmer2001 Is your site using "activity based" job triggers or "cron based"? My only guess is you're using the former, and for one reason or another the job got delayed until past midnight, and so it started counting posts made from September 4th instead of September 3rd.
This used to happen quite a lot in much earlier versions of the addon, but it's now using the same logic as XF's admin panel daily stats, and so I haven't seen it happen in maybe a year or more now. Definitely let me know if you notice it happening again though.