The difference is caused by id="content_main" which exists in one of the pages and not in the other. It is absolutely positioned and has no impact on the flow.
In the other page you have div with some breaks in them which follow floated content and therefore it's background rises up to meet #container and soaks the negative margin up that you placed on #container.
The other page has no such static content and the negative margin hits the right floated element instead.
Be careful with multiple floated elements like that. If you want a side column then just float one column and then stack the content inside otherwise some browsers won't allow elements to rise pass a series of floats unless there are no gaps between them.