For that you need "clean content" to be indexed by the GSA.
What you can see in your example is that parts of the general navigation are part of the summary.
You should implement "googleon / googleof" tags in the source of your HTML. By using that you can exlude parts of the HTML from indexing by the GSA. That way the text in navigation menu's / headers is not part of the text that the GSA indexes. Thus that part of the page will not be used for creating the contextual summary (keyword in context) of the GSA.
I don't know how Google.com takes care of that, but I know that they also use the googleoff / googleon tags (if the documentation is correct).