One of my client required me to use plagiarismdetect to check the articles.I did it also with my other costumer but when I check it did not pass copyscape but it is 0% plagiarism in plagiarismdetect. Is this plagiarismdetect reliable? I doubt. Have you tried using this too?
I believe plagiarismdetect.com uses Google to check for duplicate content. Copyscape may use additional search engine sources. You can also copy-paste snippets from the article into Google et al.