Missing pages
In this comparison, “missing” means a URL was found by the available crawl but was not listed in the observed sitemap set. It does not mean the page is missing from the website or a search engine’s index.
Decide whether inclusion is appropriate
Open the URL and confirm its response, intended canonical address and indexing policy. A useful public canonical page may be a candidate for inclusion. A redirect, duplicate, private page or deliberately excluded URL may correctly remain out of the sitemap.
Check whether all relevant sitemaps were read and compare equivalent observation times. An unavailable child sitemap can make an apparent omission misleading. Use the editor to prepare a correction only after confirming intent with the maintainer.
Adding a URL to a sitemap does not guarantee indexing, create the page or change its noindex policy. See Pages evidence and sitemap versus crawl.