Sometimes I see posts popping up in Google Reader and when I try to click the hyperlink to go to the original post, it's already deleted.
How is it possible that Google Reader still shows posts that are no longer available?
It scans RSS feeds, and at the time the feed was scanned the link was still present in the feed. Google has no control over the content of the site that it links to on the reader and does not know if the link is dead.
Basically, it will show an entry if it was present in the feed and the time it was retrieved.