Right, because the content area may be able to expand; it is going out as far as the container div I am guessing. However, the container div can't go very far in because the 2 divs in that design (I know isn't yours (h1 and .leaderboard - among other fixed widths in the code)) can't collapse in.

For things to be fluid or flexible, you need to use percent measurements all the way through.

Maybe thats still not your problem... But I think that is what is causing it to break up. (and would cause any other design to break up as well)