Because wordpress.com is hosted in the US, and the .com domain name is international, Google has no clue what nationality/country you are from. Of course language is an indication, but a lot of languages (English, French, Dutch, German...) are spoken in several countries.
If you don't live in the US, most of your compatriots will probably use the localised version of Google (google.fr, google.nl, google.de...). Your results in your local search engine will improve dramatically if you indicate your location/country in Google Webmaster Tools - here's a writeup:
http://omtersaaist.net/2007/11/02/claim-your-wordpresscom-blog-in-google-webmaster-tools/
(BTW: the possibility of indicating your country is really new, see http://searchengineland.com/071030-232502.php and made me search for a way to claim my wordpress.com blog. I was so proud I found a way and I wrote my blog post ;-) , only to discover via the forums that several people already had discovered this long before me :-S
http://jalaj.net/2007/05/21/get-your-wordpresscom-blog-google-verified/
http://tigredefogo.wordpress.com/2007/05/08/how-to-make-google-verify-your-wordpresscom-blog-verify-site-ownership/
Yet, now you have one reason more to claim your blog in Google!