I think it's because it has to be 70% of something. The only thing bigger than the body is the html, so if that hasn't been set, then you're left with "70% of whatever the browser feels like" ;)
Height is always trickier than width, because html by its nature is vertically scrolling. That means its height is whatever it needs to be, while the width is determined either by content or by user's window-- generally whichever is bigger.
Food for thought, there. If html had been developed by people who used vertical rather than horizontal writing, would web pages have a fixed height combined with unlimited horizontal scrolling?