Recently I see a strange thing may be other already know it. Seems like Google keeps multiple copies of same pages with different times of update and use them in SERP.
When doing site search "site:exmple.com" when I see the snippet description of the page xyz.html and compared it with "site:example.com/xyz.html" they were different in fact the later consists many months previous page. To verify this I compared the Google's cached pages those were also different. What is the use of having older pages if they will be shown in SERP visitor will not find exact information when he has reach to the page?
The snippet database comes from somewhere else other than the cache copy of the whole page that you see when clicking the cache link.
When searching for words that return some page that used to have those words on, perhaps several or many months ago, but no longer contain those words, I see the page returned as a Supplemental Result and see the words in the snippet, but do not see those words anywhere in the dated cache copy which will usually be only days or weeks old.
The snippet database often contains content that hasn't been on the site for many weeks or months, content that is no longer in the Google cache copy either. It has all that, as well as content that is current.