You most likely have indexed pages that are not included in your sitemap.
Google Index -> Index Status - shows all pages that are indexed for your domain.
Crawl > Sitemaps - shows which pages were indexed from the sitemap you submitted
This is normal. However, if the number grows quite large for a small site you might have to plug a few holes using your robots.txt file. Here is some sample code:
Also, keep in mind that GWT is not very accurate and information doesn't get updated at the same time.