Are you duplicating the media files on your site? I see 2 media files with the same ID. Sitemap only shows the posts which are indexed. On your site, all the media files are set to noindex. When I set a media file to the index, it shows in the sitemap, when I view and edit it, it again becomes noindex. After further investigation, I found that there are 2 media files with the same ID. So basically duplicate media files are created on your site. Please see the screencast I added in the Sensitive Data Section.