When you send to multiple groups, the Platform automatically de-dupes those groups for you and aggregates the results on the Message Summary page. After our August 2014 feature update, you’ll be able to drill down into the results by Group, similar to how the Domain report works:
This can help you see if one group is engaging drastically differently than another. Are your members the ones doing most of the clicking? Perhaps you can find your Inactives and compare your Inactive and Actives results to prove that the Inactives really aren’t engaging. Or if you’re curious which list had the high bounce rate, you can see that, too.
It’s possible that you have Contacts that would in multiple Groups (i.e. attendees to this month’s event and attendees to last months event). Because the Platform de-dupes your lists for you, they’ll only get one copy. However, their results will be tallied under each Group of which they’re members. So on this tab, you may see numbers that are higher (if you add them together) than on the aggregated report.