I think Firingsquad looked into this in their 4 day hands on review of the 360. It was due to all the demos and media that ships on the 360 by default. It's not that only 13GB is accessible. It's that there's already 7GB of data on the HDD when you get it. But it does beg the question, with more downloads to come, how are users supposed to keep up with only 20GB total?