In Google webmaster tools, you can have a different number of submitted urls comparing to the indexed urls.
You can have more indexed urls if Google has referenced pictures for example.
But most of the time, you will get less indexed urls. Maybe because you have a board, and Google will only reference the first page of each thread instead of every page, or every posts. Why ?
For the page, it may find that every page has the same content. For the posts, it won’t reference its data several time, it would be like having duplicate data.
So don’t worry if Google doesn’t reference all the pages generated in your xml sitemap.