Google is trying to show relevance in its search results. When someone looks up "little neck clams," it wants to show that yes, this page covers "little neck clams." So the excerpt it shows will be the first instance of "little neck clams" in the article.
Google uses the summary -- the default description -- only if the page generally matches the search query, or if the search seems to indicate a more general intention behind the search. For example, "restaurants Little Saigon" isn't necessarily a search for a webpage matching the exact phrase "Little Saigon," it's a search for good restaurants near the searcher's location. In that case "Little Saigon" isn't as important as a general description/summary of the restaurant, so Google is more likely to show the default description/summary instead of an excerpt from further down on the page.