Ensure that the Report you are viewing for your mailing has been updated to the most current statistics:
  1. Navigate to Communications > eMail
  2. Hover over your Campaign Name and click Manage
  3. Hover over your eMail and click Reports
  4. Click the Update Report link at the top and confirm you want to update
  5. Back out of the Reports screen and go back into it
  6. Ensure that the "Results as of" lists the current date and time

From here you can click "View Report" next to "Total Emails Sent Successfully".  (Keep in mind that Total Emails Sent Successfully are only those that were able to be sent, and would not include any email addresses currently on the Kintera Blacklist, unsubscribed from your mailing, duplicate email addresses, etc.  You may want to also check these numbers to see if this could account for your missing Receivers.)

If it still appears, based on the report statistics, that the total number of emails is different from your Receivers query:
chat with Support and reference this article. 

Please include:
  1. Campaign Name
  2. Mailing Name

We will take a look to see if we can identify why these numbers are off.