There seem to be a few differences between the data that’s displayed via the Graph and Table views for the admin panel reports, and the version available for export via CSV.
This example is from the Table view of
/admin/reports/posts, which seems to be including the date immediately after the selected range (April 1st, instead of ending on March 31).
The Graph view of that page shows the same range, also ending with “zero posts” on April 1.
However, the data export for this range doesn’t include the additional date, ending on Mar 31, with a value of zero for that day:
It’s also worth noting that the last day in the range (both the “extra” day in the Admin/Reports view, and the actual last day from the CSV export) always seems to show zero posts/topics. Selecting a broader range shows that there actually were posts made on that day, though: