They may have no sitemap.xml, a sitemap.xml that only has "important" pages or a sitemap.xml that only has higher level pages and they are relying on the bots to find deeper pages from there e.g. an ecom site that has sub-cat pages listed but not product pages because links to all product pages are on the sitemap listed sub-cat pages.
From what I have seen, it seems as though a URL being on the sitemap.xml plays a part (but not the whole part) in it getting shuffled out. I would swear it looks like G is saying if you don't have enough time/incination to bother with a URL, neither do we. ;)