Turnitin does not check to see if a piece of work is plagiarized or not. 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.
It is perfectly natural for an assignment to match against some of our database. If your student has used quotes and has referenced correctly, there will be instances where we will find a match. The Similarity Score Index (SSI) simply makes you aware of any problem areas in a student's paper; you can then use this as a tool as a part of a larger process, in order to determine if the match was acceptable.
For example, your student may have submitted a paper to Turnitin previously in the past - if they had their name on that submission, it is entirely possible that, if you are not excluding small matches, you might see this on their Similarity Report.
Another example may concern a student copying and pasting a chunk of text into their paper due to a lack of knowledge on the topic they are covering, their Similarity Index is 10%. This is compared to another student who may have a firm basis of knowledge for the paper and knows enough to gather information from several sources to quote, their Similarity Index is 12%.
Both students will be shown to have matches against our database. However one of these students copied directly from a website where as the other properly sourced their quotes. We suggest using the similarity report as an indication that plagiarism may have occurred and then use it as a part of a larger process to determine if these matches are acceptable or not.