Hi hshojaee --
I understand you question and your concern and your confusion -- theapparatus explained the current condition -- but I understand your want to burn your feed and have only one RSS feed for your blog and not two.
On TypePad and Blogger you can choose to use FeedBurner as your sole RSS feed service.
On WordPress.com you cannot make that choice and with this announcement a couple of days ago:
My sense is you will never be able to use the WordPress.com backend to burn your RSS feed through FeedBurner because you can set up advertising on your FeedBurner RSS feed to deliver money into your hands and WordPress.com isn’t about that as a free blog service.
I presently have the same issue on my WP.com blog. I offer a giant RSS feed icon for reader to use that I manage with FeedBurner -- I like the FeedBurner interface for management and deep analysis of subscribed readership -- but if my readers click on the natural location of my RSS feed, or use a browser’s “auto-discovery” feature, they'll get the WP.com version of my feed.
There isn't anything we can do about that now -- and since the new WP.com RSS enhancements are similar to those provided by FeedBurner on a basic "social networking" level -- we may not have a choice any time soon to provide a single RSS feed through FeedBurner.