For one of my sites, the number of "Indexed URLs" in Google Sitemaps is 2374. However, when I check using the "site:" operator, I see only 14 pages. So where exactly are these "Indexed URLs" ???
I have seen a difference between site: stats when using the www vs not using the www on the front of my URL. Either way though, I never have the same number on Indexed Urls and site:
Google crawls many urls. However it indexes certain ones based on the content and such and depending on the importance in your sitemap. If you start to get more backlinks, you will notice the amount of pages indexed by Google will increase.