Turnitin's Similarity Report is NOT an assessment of whether a paper has or has not been plagiarized. Similarity Reports are simply a tool to help an instructor find sources that contain text similar to submitted papers. The decision to deem any work plagiarized must be made carefully, and only after in depth examination of both the submitted paper and suspect sources.
Please watch this mini tutorial from Turnitin to understand the OriginalityCheck technology and how to access and interpret Similarity Reports:
You may also want to watch this more in-depth recorded webinar.