My paper is completely original. Why is my similarity score so high?

The similarity checker matches your text against an extensive database of over 99 billion current and archived web pages, 1.8 billion student papers and 89 million academic articles, and flags portions with high similarity to existing sources. If it finds instances where a piece of writing has a high match with an original source, this will be highlighted for you to review and take the necessary measures. Given the nature of language and the way the tool is designed, some similarity is not unusual. You could rephrase the flagged portions to reduce the similarity score. If you find the linked source relevant to your work, you could even add a citation.