Transfers are also a reason for this. Depending on how transfers are performed, and if the agent receiving the transfer is in the same workgroup, it will cause the numbers to be off as well. You can have multiple agent segments for an interaction, while only having 1 queue segment. Bumping up one level, the same can be said for multiple workgroup segments, while having only 1 DNIS segment.
Ensure that Track Routing Exceptions is enabled in the IA -> Interaction Tracker -> Items Tracked tab. Doing so will allow you to go into the EE_TransferEvents_viw view in the database for more information on transfers and how they may be impacting your OOB reporting data.