That's not a circle, that's a grid of pixels being colored in. It's a lossy representation of something else.
A circle is a shape which is a graphics primitive. Graphics primitives can be moved, transformed, skewed, etc... with no loss of information.
If you do that with a grid of pixels, you're going to lose information.
Edit: Five downvotes. Apparently people don't know how to use vectors in bitmap authoring software, or why selections are the lossy way to create shapes. See details in my comments below.