Hi Matt,
Inbound/outbound means that a conversation had multiple parts, and some of them were inbound while others were outbound. A number of situations could cause that. Here's the definition from the image map in the article about that report, which has an example https://help.mypurecloud.com/articles/interaction-details-report/:
"Inbound/outbound indicates that the conversation had multiple parts that were of opposing directions. For example, an outbound call to a queue's external IVR contains both inbound and outbound segments."
I think that you could also look at the timeline tab for a specific interaction to get a better idea of why a particular interaction is listed as inbound/outbound: https://help.mypurecloud.com/articles/interaction-detail-page/