I didn’t complete the Task that was assigned to me, yet its status is "Complete". Why?

This article describes how a Task may be complete without you personally completing it.

Tasks can be sent to just one worker or multiple workers.

When a Task is assigned to multiple workers only one worker needs to complete it, and the Task will display "Complete" for everyone. Clicking on the Task in question shows you Task Details, including who else was assigned to this Task.