A campaign can be set up to send to more than one segment or category. It is possible for the same contact to be a member of one or all of those segments or categories. When the campaign is sent the system will dedupe contacts that appear in multiple segments.
This function is called cross post merging. Cross post merging is a system default that prevents contacts from receiving the same email more than once in a 24 hour period.
Whenever you see a cross post merge messages in a sent campaign log, it indicates that duplicate records were found and removed.