I would assume so it aligns all the way to the left of the page, and then it's 100% since the menu expands and contracts based on the window size of the browser... so the 100% makes sure to always use as much space as the window allows.
~Sovereign
Hello, i just read the sliding doors technique
http://www.alistapart.com/articles/slidingdoors/
which is awesome, and i did understand everything but, why does the header div needs to be floated left and have a width of 100 %
I think the answer is pretty easy, but I can't figure it out.
Thanks, Marcelo.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic