It is possible that there are appoinments/returns on your tablet that were modified, but eventually weren't taken in/delivered on the same device. When a clean up happens the application will look if these were already taken in/delivered on PlanManager. If this is the case they will be deleted from the device. 

The Reception app will do a clean up automatically one time per day. The user can also force this by clicking on "Clean up" in the menu of the overview screen.