Border corner shapes, just a thought.
Prompted by Lea Verou's post: Preview…
The "tails" sticking out of the top corners of the image in the lower left are artifacts of saving from Illustrator. The actual shape is tear–drop.
Here's a PDF copy for comparison.
In the case of the top 2 images, the "inner", or third anchor point would be assumed to have neutral values when it's values are not specified.