Why does Content Aware Fill leave a small white line around the border of the selection?

So I want to use Content-Aware Fill to cover up a football in an image. It works great, mimicking the texure of the field behind the ball and making the ball completely invisible, but for some reason it leaves a thin white line where I made the selection.

What exactly I am doing:

1.  Opening the image
2.  Using lasso select to make a selection around the ball and its shadow
3.  Pressing the delete key to erase what is in the selection
4.  Right clicking, and then clicking "Content-Aware Fill"
5.  Making sure I'm happy with the paramaters and the results, and clicking "Apply"
6.  And, lo and behold, for some reason, a small white line is left exactly where I made the selecttion. Keep in mind that I no longer have an active selection, and that this line is baked into the pixels themselves, and is not just a rendering glitch. It stays there when I save the image, too.

Does anyone know what is going on?