Google: Sitemaps Do Not Guarantee Indexing

This is likely obvious to most the readers here but it is simple, submitting an XML sitemap to Google does not mean the pages in that sitemap will be fully indexed.A Google Webmaster Help thread has Google's Gary Illyes responding to a question about why a site that has submitted 40,000 pages only has 100 pages indexed in Google.
For example, here are two sites that have submitted their URLs to Google via an XML sitemap file. One, has submitted 17,987 pages and Google has actually index all of them, plus one. :) The other has submitted over 7 million pages, but Google has only indexed about 4 million of them, which is about 53% of the pages submitted.
Why did Google index all the pages on one site but only about half on this other site? Why did Google only index 100 pages of the 40,000 of the site complaining above?

Google December 19th Ranking Fluctuations Reported

So Google confirmed they reduced the authorship snippets from showing in the results. We know that. Matt Cutts strongly implied that there was no update on December 17th, despite all the tracking tools lighting up on that date. That implication that Google minimizes the algorithm updates before the holidays should apply to what I am seeing today - a lot of chatter, in some niches, of a Google update.

The key indicator I use is webmaster/SEO chatter. I check the chatter atWebmasterWorld and dozens of other forums and the chatter has picked up yesterday. Martin Ice Web, who is a Senior Member at WebmasterWorld but is based in Germany, is the loudest on claiming updates today. There are many who agree and see major changes and there are many threads in the Google forums with individual complaints.

That being said, one of the tools I rarely show you, because it often doesn't match the other tools, is the DigitalPoint Keyword Tracker averages changes. It is something fairly new added to the forums sidebar and this reports changes of actual rankings for hundreds of thousands of sites and I'd say millions of keywords. It is based off what webmasters enter into the tracking tools.