This trick won't work if you use Google to check.
No other software comes close to Google in finding copied content, you only need to know how to use it. Verbatim search and "double quotes are your friends".
Actually, it doesn't pass Google as well. But it works both ways really. The content doesn't rank in Google either for the intended keyword, since Google peruses it to be a completely different input.
Being in the content business, I've come across this sham more than once.
About six months ago, I was contacted by a fellow BHW member who wanted me to develop a content strategy for 3 websites he planned. I did the same, and sent him a detailed spreadsheet complete with SILO details and everything else. He paid me for the content strategy, but found a better deal for the content somewhere else (outside BHW) and politely parted ways.
A month later, he approached me stating that his content was done and he'd like to seek my help in getting the sites setup. On further discussion, he said that he had ordered about $3000 worth of content for 3 of his websites. And then proceeded to send me 3 zip files each containing individual word files for each of the topics that were included in my spreadsheet.
It was only when one of my staff members was uploading the articles, did she realize that some of the content seemed very familiar. She had actually written that article herself about a month ago for a different client. When she compared, it to her archives, the article was indeed her own. The original article was indexed, but yet showed up as unique in Copyscape, even Google was of no help.
Further investigation led me to this. I had heard of this trick before, but never really seen it in action. And then, as expected, every single article was using this exact same trick.
When the client was informed, at first he couldn't believe it, by the time he realized this, he almost had a heart attack. He had paid only $500 via Paypal, which he could chargeback. While the remaining $2500 was paid via WU, since the other person (in Philippines) had claimed Paypal was having issues.
It took us a LOT of time, patience and hard work to help the client get through this. This could've broken the man, but fortunately, he survived. Still, no idea why people do this. This is the same as robbery. Plain simple robbery. Might as well break into his house and take all his belongings.
Pro Tip - If you're on a Mac, open the article in TextEdit - Mac's built-in text editor. If the article is using the above trick, you should see a whole lot of red underlines. That should tell you that the article uses this trick.