This is actually the place for the webmasters.stackexchange.com.
But to answer your questions: Google trying to show the most relevant results based on what the search query is. This also includes the meta description. Google may choose to show another content on your page which is different than your meta description tag.
Google's attempts to match snippets to queries don't always work the way you'd like, but in general they're a good thing. Matching, bolded keywords drive click-throughs, and people rarely read the whole text of a snippet. If it’s just a couple of long-tail queries, don't worry about it.