Have you ever attempted to rewrite a PLR article to be used as a blog post in your blog? I guess you had attempted at least once. Since you rewrote it sentence by sentence, you were 100% confident of its uniqueness and thus never bothered to check with a duplicate content checking tool.
Fine! When, next time you do that, ensure you don't repeat four words from the source consecutively. I read this valuable tip in the Internet.
In addition, use Jet Tracker duplicate content checker to test the % uniqueness of your spun article. It is a good tool and it is free. It can also check 40 articles at a time. Hmmm... versatile really!