CSS - 出现在错误元素上的nth-child条件

The goal is a rounded side nav with hover effects in CSS. I'm using nth-child to control border radius for the top and bottom. As you will see on the fiddle, when you hover over the elements in between the 1st and 4th child, you get the radius settings of nth-child(1). Why?The goal is a rounded side nav with hover effec