Turnitin does not check for plagiarism in a piece of work. Instead, we will check a student's work against our database, and if there are instances where a student's writing is similar to, or matches against, one of our sources, we will flag this for you to review. Our database includes billions of web pages: both current and archived content from the internet, a repository of works students have submitted to Turnitin in the past, and a collection of documents, which comprises thousands of periodicals, journals, and publications.
It is perfectly natural for an assignment to match against some of our database. If a student has used quotes and has referenced correctly, there will be instances where we will find a match. The similarity score simply highlights of any problem areas in a student's paper; you can then use this as an investigative tool, in order to determine if the match is or is not acceptable.