I've just noticed something for the first time, and I think it might be accidental.
When you decide which categories your post will belong to, it says so in the post. But should you click on these "Categories" from the post, to directly see other posts in the same category on the same blog, instead you will be taken to the "Tags" page of wordpress.
Regardless whether you click the "Photos" tag or Categories, if you do so from the top of a post, you'll be taken to http://en.wordpress.com/tag/photos/ . where if I'm on a specific page, I expect the Categories' entries to take me to categories, even if not done from the widget.