My b2evolution Version: 4

This is just a question to satisfy my curiosity but I find it odd: I'm wondering why changing the body background color in the CSS for Hot Air Balloons would effect the rounding of the corners. If you look here: http://operationsomething.com/blog2.php you'll notice the upper black corners are rounded. However, when I change the background color the corners are no longer rounded. I can fix this by using the border-radius function but it seems strange the they would be effected. What's even stranger is only certain colors make this happen. Any ideas why this would occur?