Google's cache of your page is the last updated version of your page correct? So if you recently updated a page, Google wouldn't be able to include any new content in their searches until they updated the cache. My question is, how often can you expect Google to update their caches of your pages, and what can prompt them to do so? Also is there anyway to make them cache your pages faster?
The more PR your site has, the more frequent the cache gets updated and the more deeper your site gets indexed.
If you continually update your site then Google will send spiders to your site more often and update your cache more often. Also if you are an authority site or have a high PR and lots of links, the chances are your cache will be updated more often. For examlpe CNN is probably updated every hour or so.
Try submitting a Google site map if you have a big site and use WebMaster central to check if you can increase the crawl rate for your site