Why does the prayer rug blacken and show wear at the spot of prostration, even though the body's weight during bowing is on the knees and the sitting period is longer, yet the knee spot is not affected to the same extent?

The reason for the darkening of the forehead might be due to the wetness from ablution remaining on it. When a person prostrates, the place of prostration becomes wet, and with repeated instances, that spot darkens, unlike the knees. This issue does not entail any [specific religious] action.