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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?
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.
abhishekkaushik did mention that in his case, he was seeing two different cached pages. But I can see how the cache might not always be in synch with the description snippets.