How accurate should plagiarism detection be?
It depends on your needs.
If you are a publisher and need to make absolutely certain that the manuscript you are about to purchase is indeed the intellectual property of the seller, then you’ll want a very high level of confidence in the plagiarism process.
If you are an instructor in a course, you probably have different goals. You want your students to author responsibly, to understand and respect the intellectual property of others and of course, to avoid the sanctions of the campus judicial policies on academic honesty. The presence of a plagiarism service in the assignment submission process serves as an incentive. If most forms of plagiarism are flagged for inspection by the instructor, these goals will be realized. As in almost anything, achieving 100% certainty is quite difficult and expensive, if possible at all. Debora Weber-Wulff, a thought leader in plagiarism detection, has said  “…it is not important in an education setting to find all of the plagiarism in a paper. It is sufficient to find enough for a sanction to be necessary.” Sanctions are perhaps a last resort, but engaging the student in a discussion about academic honesty comes first.
 Weber-Wulff, et al., 2013. Plagiarism Detection Software Test 2013. http://plagiat.htw-berlin.de/?attachment_id=2916