Why are HF scatter signals in the skip zone usually weak?

The skip zone is normally a dead area between the maximum reach of a ground wave and the closest point where a skywave bounces back to Earth. Scatter propagation can occasionally fill this gap. When a signal hits irregularities in the ionosphere, a very small percentage of the RF energy is scattered backward or downward into the skip zone. Because this is not an efficient, mirror-like reflection, the vast majority of the signal energy continues on its primary path or is lost. As a result, the fraction of the signal that actually makes it into the skip zone is minuscule, resulting in a very weak signal at the receiver. The weakness is due to this poor reflection efficiency, not absorption by ground waves or ducts.