I think it has to do with how the brush was defined. How much the brush has been scaled from the size it was defined at. How Photoshop lays down paint and perhaps some math rounding. When I use Photoshop predefined hard square brushes. I see blue squares like you posted for a single mouse click. Click a few more times without moving the mouse I see the right and bottom soft area fill in more.
So I did some testing opened a 1600x1200 new document and layed out guide line for 400x400 px squares and guides to locate their centers. I then defined a 395 px Square brush I selected that defined brush to use set the paint to blue and click once on the center of the 400px squares. Here is how CS6 worked for the brush with no scaling.
Defined a 10PX Square brush scaling it to 395 and doing the same produces soft right and bottom edges.....