Only at the very beginning of the development cycle of a Ubuntu release are they strictly the same. Then Ubuntu has a development cycle where they break packages. At some point in their process they have a milestone called DebianImportFreeze. From this point on, packages will start to outdate in addtion to being actively broken.

Edit: instead of writing "strictly the same," I should have written "compiled from sources which are stricly the same." While Ubuntu sources packages start as unmodified Debian source packages, they are compiled differently and are binary-incompatible.