It's not soft serve. It's softcream.

One of the many things it will be hard to eventually leave behind in Japan. You can get your average soft serve here as well but you can also get this delightful treat. Looks like soft serve, but the texture is richer and creamier (and dreamier). It tastes more like a frozen custard or frozen pudding. I've tried mango, black sesame, vanilla, chocolate, grape, and matcha. Not all at once, though. All flavors equally life-changingly amazing. Check out the wiki entry for soft serve and there's a little snippet about soft cream in Japan.
http://en.wikipedia.org/wiki/Soft_serve