Drill downs on Dashboard

Summary:

Why does drill down report different numbers to dashboard when its the same report?

Content (required):

I've created a dashboard report for absence reasons, but when I drill down on the absence reason/number, the values are different on the drill down.

I tried to use the same dashboard report as the drill down to see where it was failing, but, even though the dashboard and criteria are identical now, the drill down shows 1 absence less.

The criteria is end date is null or between 2 dates (our absence year), and its the null record that's not showing on the drill down which is bizarre as its the same report (for testing purposes).