Looking at my own site and having configured the SEO, I can say that the search descriptions displayed on Google come from one of two places:
- The written description configured by the person who made the site
- A snippet of text on the site from the last index
The most likely reason for the description text to be wrong is that Google is using a snippet from the site, but the page has changed since Google has last indexed it. This is especially true on blog homepages where it is updated on a daily basis, but Google indexes only every couple of months.
Another option is that the author put up a 301 request and redirected the page to a new page. The old description and link still float around Google, but will actually lead to a completely different page which may or may not be correct.
The last option is that the page simply no longer exists. When this happens, Google will still try to resolve the request if it can find a similar page or it might go one page up (example.com/about/staff can go to example.com/about)
Basically, if the content is changing faster than Google is indexing it you can get a wrong result, especially if there were major changes.