These tools work directly and only on the database in a limited way.
When you save your page, they mainly analyze four fields (title, body of text, the tag title and the tag description) of your article.
If you have customized fields, they cannot read their contents.
Often the pages displayed are more complex than the only information encoded in the form of your page.
You have a header, a footer, one or two sidebars, and navigation menus.
Sometimes a template display will look for content in other pages: for example a list of suggested articles to your visitors (pictures, titles, descriptions).
All these elements add "words" and "tags" that SEO plugins are not allowed "to see".
They are "blind".
So their analysis on the keywords is incomplete, even fanciful.
Sometimes they can even analyze content which is not displayed on your site.
And in some cases, your page displays are harmful to your SEO because they create a real semantic pollution.
You have the same groups of sentences that are repeated throughout your site.