It forces the form to the width of the content div.
The problem is that the form is wider than the content div. So the it is being pushed down the page to a place where there is enough room to display it.
By adding the overflow: hidden property it tells the browser to just not show the part of the form that overflows the boundary of the content div rather than push things down the page.
You either need to include the overflow: hidden property, or change the width of the form so it will fit within the content div.