... becasue they have MU installed in a folder called "blogs".
it's pretty easy to make the main page look like it's part of the same MU install. Done it myself.
http://wanderlustandlipstick.com
http://wanderlustandlipstick.com/blogs/
in this case, there's a WP install on the main site, and a MU install in the /blogs/ folder. I'd bet money the site you're looking at is setup the exact same way.
Edit: and a check reveals I am correct. Look at the source for the main page - it uses one theme. looks at the /blog/ page. It uses a completely different theme, even though it looks the same. Why? Because it's not the same "blog".
Dump WPMU in the /blogs/ folder. Put WP in the main folder. Slap a copy of the same theme on both. User will never be able to tell.