Derek,
From the field descriptions in the Reporting User Guide, it would appear that "Router_Queue_Interval" is used for caller data related to the Precision Queue (Service Level, calls queued, abandon in queue, answers per step), while "Skill_Group_Interval" is used for agent-specific data (handle time, handle count, etc). There are some definite issues with the Schema when looking at items related to Precision Queues... for example, the "Router_Queue_Interval" description for "ServiceLevelCalls" is not accurate at all.
In this case, I would trust the Reporting Guide.
-Jameson