Content column
The point of this exercise is to see how to make full length columns, or, what is the the same in these matters, the appearance of full length columns.
If we give all the main players, the wrapper DIV, the nav DIV and the content DIV some background colours we can see what the problem is.
Both of the two containers share the same CSS and differ in the HTML only in that the second one on the page has many more list items in the left column than in the first.
In this example the navigation falls short and the container's red can be seen, in the next one you can see the content falling short.