In Knowledge Classic, why would an article showing as "not published" still appear in a search?

Summary:

Why would an article that is showing as "Not Published" still appear in someone's search and allow them to open it?

When going into "Authoring Classic" and clicking on "Not Published" for said article, it gives the option to click on "Unpublish" as shown in the screenshot below. What is the difference between "Not Published" and "Unpublish?"

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Version (include the version you are using, if applicable):

Knowledge Classic

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