Welcome to WebmasterWorld Guest from 18.104.22.168
All I know about cache and cache date is when I used to be in G's good graces they would reindex about 10-20% of my pages every night and the next day those pages would have the cache date next to them.
As for why some entries dont have any reference to cache is a puzzlement to me too.
Now I'm lucky to see new cache dates every 4-5 days.
The green date is a "fresh date" which appears by a page for a couple of days when Google has a fresh snapshot of the cache. My home pages, for example, always have a date, since Google calls by every day. Pages deep down generally don't have, and the date of the cache (view the cached page to see that) is typically 2 weeks ago.