Assuming we are looking at the same results, the pages at position #6 and #10 are not copies of the stackoverflow content at position #8. They are copies of http://stackoverflow.com/questions/1399293/test-priorities-o.... Unfortunately, the only place that the word "delay" (which is in your query) sometimes appears on that stackoverflow page is in the "related" links in the right column. At the time Google last crawled that stackoverflow page (see the cache), "delay" wasn't on the page, only "delayed". Whereas, the last time Google crawled the other two pages you mentioned, they did have "delay" on the page. Google should still be able to do better, but this little complication certainly makes things more difficult.
One UI issue we've struggled with is how to tell the user that there isn't a good result for their query. This comes up when we evaluate changes that remove crap pages all the time. For nearly any search you do, something will come up, just because our index is enormous. If the only thing in the result set that remotely matches the query intent is a nearly empty page on a scummy site, is that better or worse than having no remotely relevant results at all? I definitely lean towards it being worse, but many people disagree.