i see this all the time and i never understood it. we make backup copies from disk to tape, both attached to the same media agent. During these aux copies it has the Data Transferred over Network number, which I think should be 0, but there is usually a number there. For example this aux job has these numbers, still running:
Total Data Processed: 3.23 TB
Data Transferred Over Network: 107.95 GB
Total Data to Process: 4.7 TB