Task Reports 100% Complete But Not Actually Finished

[Stoaty]

I set up a task to create a full image of my C: drive on drive E: (external USB) and replicate to drive G: (external USB Drive). When I executed it on the spot I waited and noted that it was 100% complete. However, the replication was still running so the task wasn't actually complete. Why the lack of accurate information? If I hadn't been paying attention I might have shut down my computer thinking I had a full image backup on both of the external disks when in fact only one had completed.