This is because you have sent an email that has already been sent before.
You have sent this email to a new group of contacts - this has then affected the report statistics.
What you need to do next time is to duplicate an older campaign that has been sent and then re-name the campaign.
This is to ensure that no report statistics are associated to the new campaign.
You will need to edit the duplicated email template and send this out - this will be the first time the email has been sent then so you will now have accurate report statistics.
Please click here to see how to duplicate your email campaign.