Google Caches only HTML content
I believe that Google only caches the HTML content and additional files like JS/CSS/Images are still required to display the cache correctly. If you was to rename an image or css file completely you should notice that cache alters as well. But you can't change the HTML Content no mater what you do until they refresh their system.
A quick test
A quick test on this theory would be to quickly rename your CSS file breaking it temporary and quickly refresh the cache page and see the result. CTRL+F5 (Hard Refresh).
Don't Worry About Google Cache
I wouldn't concern yourself to much on the Google Cache its not really important, it's useful for using when sites are down or when deletions occur you can recover a lot of the HTML data. In terms of rankings, no effect whats so ever.
Cached: Google crawls the web and takes snapshots of each page. When
you click Cached, you'll see the webpage as it looked when we last
The above snippet from the Google cache link may imply that its when your page was last indexed this is simply untrue, the cache service Google offer is often days behind the algorithms itself. And a prime example of this is when you post something in your blog or create another page on your site your often see that your page is indeed indexed yet there is NO CACHE PAGED - Which is a clear indication that the cache system is not real time as the algorithms.