When you email a distribution list using overall tracking, you may notice that the recipient count registers as "1". This is because it is sent to the email address and does not "see" the number of people in the list as a part of the process.
However, you can change it manually so the recipients are accounted for in the metrics.
To change this, hover over the "recipient" count until you see the pencil icon, and edit the recipient list.
After changing the number of recipients, refresh the page, and your data in percentages will be updated accordingly.
You may also want to consider changing the expansion method: List Expansion: EWS vs Graph Utility Account, Improving Recipient Count Accuracy
See our video on for more information