Google Reader caches the feeds that you subscribe to, not from the time you’ve subscribed to it, but from the time the first person (to ever subscribe to that feed) subscribed to it. I am guessing this works the other way around, with the feed remaining in the cache until there are no subscribers left.

This is annoying because updated posts are not updated in the cache — I don’t think Google Reader likes or understands the RSS last modified tag. Deleted posts are not removed from the cache either, so be careful of what you put up, in case you make a mistake. You could use this as a positive though. And if you ever want to remove all trace of your blog from the Internet, you can’t; unless you track down each and every one of your subscribers and make them un-subscribe to your feed.

Guess they can’t get everything right.