Search engines - in order to improve the experience of their users -, have been creating snippets based on the searched term and also based on what they understand as more appropriate for the content that the page in question displays. According to Google itself:
"The generation of titles and page descriptions (or "snippets") by Google is fully automated and considers both the page content and the references that appear on the web about it. The purpose of the snippet and the title is to represent and describe each result the best possible way and explain how it relates to the user query."
That is, both title tags and description tags may or may not be used by the search engines. The analysis is performed automatically by the robots of each service, and they can judge that the tags are appropriate and thus use them, but also create a snippet of their own based on other content on the page. For more information and details, we suggest the following Google article: