A WebmasterWorld thread has two claims from respected members, giving us a little more insight into Google's search engine.
The first thing we learn is that Google has several cache databases.
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.
Tedster, WebmasterWorld administrator, confirmed this to be true. Is it simply a data center thing?
Respected member, g1smd, added that often the snippet Google chooses is not pulled from the cache. He said that Google has a snippet database that is typically much older than what is found in the cache.
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.
Note, none of this has been confirmed by Google.
Forum discussion at WebmasterWorld.