Quote Originally Posted by Michael R 1974 View Post
Reduced development is what reduces contrast in the negative. The reason people often rate the film at a lower exposure index when planning for reduced development, is that reducing development on its own can result in a loss of film speed and shadow detail. This is true particularly when development/contrast is significantly reduced. By giving the film some extra exposure you give more support to the shadows so that they are not lost when you give reduced development.
Good explanation.