One pixel notched corners as used by Google Analytics

"I use Google Analytics, and I noticed that their left nav has an interesting characteristic: instead of each option being boxed in a clickable rectangle, there is a one pixel notch in each corner. It's not necessarily a curved corner, but it is a little softer than a normal box. I would've naturally thought that if someone is going to use background images to create a capsule effect, they would have used anti-aliased images. But upon closer inspection, no background images were used at all."