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A. Those are called "innocent matches" or "small matches." Some common phrases are likely to occur in many articles and essays on the same subject. In these cases, Turnitin will report similarities to some online text and to student papers submitted to the system, but these similarities are coincidental and often inevitable. The only way to not have small innocent matches is if you are the first person ever to use a particular turn of phrase!
Some types of assignments are more likely to contain these kinds of "innocent" similarities, particularly those answering a very prescribed, tightly-focused question (in which many students may be expected to use very similar expressions and phrases), some technical, scientific or legal assignments which tend to employ established phrases, expressions and terminology, and textual analysis assignments, which may contain sizeable examples from the poem, novel, or passage being discussed.
Turnitin may also identify "innocent" similarities between the assignments of a number of students on the same course if they share a common essay title, for instance, or if every submission includes a departmental cover-sheet.