Why the similarity percentage generated by Turnitin is so high?

To estimate the similarity percentage, Turnitin compares the document with all texts available on its database. A high percentage could mean that you are in presence of a serious case of plagiarism or it could also be due to another reason: sometimes it can happen that the same document is uploaded on the database more than once (i.e. a thesis draft and the final version of the same thesis). In this case, Turnitin unfortunately doesn’t recognize that it is the same text, and this is why it shows a high percentage. To avoid this inconvenience, you can exclude some sources from the percentage generation.
See how to do it at this FAQ: How can I exclude a source from the Turnitin originality report?

Important: The results generated by Turnitin should always be interpreted. The software simply indicates a similarity percentage, but, at the end, it’s always the teacher’s task to understand and decide whether it’s plagiarism or not.

(last update: 16.08.2021)