SEO Sitemaps Aren't an Indexing Guarantee
Experienced search engine optimization professionals know that just because you submit a sitemap to Google, doesn't necessarily mean that your site's pages will be indexed. That important bit of information has recently been confirmed in a Google Webmaster Help thread.
Google's Gary Illyes responded to a question about why a site that has submitted 40,000 pages has seen only 100 of its pages indexed in Google. Illyes reiterated that submitting a sitemap does not guarantee that the pages referenced in it will be indexed but rather that a sitemap is a way to help Googlebot find content. Essentially, if the URLs weren't included in the sitemaps, the search engine's crawlers will have a harder time finding those URLs and as a result might be indexed slower.
Perhaps most interesting however in Illyes's response was the suggestion that Google's algorithms may decide not to index certain URLs at all. "For instance, if the content is shallow, it may totally happen it will not be indexed at all."