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...all of the 189K indexed pages are also in my sitemap
That lines up as far as I can see. Your sitemap number (368K) is nearly twice as high as the site: operator number (189K).
I'm never surpised when Webmaster Tools information seems peculiar in some way - it happens a lot. Also note that site:example.com results are getting weirder and weirder, often omitting urls that definitely are in the index - sometimes with a simple site:example.com/directory/ query.
How many pages do you think your website has? Does that line up with either of the two numbers Google reports?
(FYI I work on Webmaster Tools.) The number of "Indexed URLs" on the Sitemaps page only includes URLs that are submitted in your Sitemap(s). So for example, if you had http://www.example.com/ in your Sitemap but http://www.example.com/index.html was indexed, the Sitemap details would report 0 URLs indexed (because the one indexed != the one in your Sitemap). If your Sitemap doesn't contain 100% of your site's URLs, or if there are small differences between the URLs in your Sitemap vs. the ones indexed (e.g. one includes some optional URL parameter), then that can affect this number. It's only designed to report the number of URLs from your Sitemap that are indexed, not the total count of URLs from your domain that are indexed.
If you think there's a bug in our data, I would strongly encourage you to come report it to us in our forum (search for [Google Webmaster Help Forum] and you should find us). Please include your URL if you do. Thanks!