near as I can figure (and I may be wrong) but the bottom and right side lines denote the entire content area. the single dots on the top and left define the 1- pixel column or row that is allowed to be stretched. Remember, a line of pixels is not a 1- dimensional line, it has width and area as well.
If you have photoshop, try this. Open up any random picture and make a 1 pixel selection (either vertically or horizontally) and then use the free transform tool and scale it in the direction perpendicular to the selection. For example, if you make a 1 pixel selection vertically across the middle of the image, scale it horizontally, you'll see what I'm trying to get at.
I am curious though, will photoshop automatically recognize the black lines on the sides as defining these areas and automatically append the file name to *.9.png?