The 1-edge and 2-edge stations are being diffracted over edges in the terrain in your direction. Essentially, they are over the horizon, either due to a hill/mountain or due to the curvature of the earth.
You can receive them if they strong enough. In an ideal situation, you should be able to receive down into, say, the single digits in noise margin. But this would require a roof mounted antenna pointed directly at the transmitter, and a sensitive (ie big) antenna. Obstructions (trees, the building walls,, etc) and local interference disrupt that ideal situation. There can also be local nulls that you simply have to work around - see here:
http://www.hdtvprimer.com/antennas/siting.html