The sky colors of the landscape are closer in lightness to the Blend color and are beginning to show through the Blend color. Lets take a look. Blend color Blend results Notice how, in the above example, the trees are now beginning to show through the Blend range. Not only are some of the colors in the trees lighter than the Blend color, the colors in the sky and grass are lighter, so the Blend color over those lighter underlying colors have less opacity. You can further alter the opacity values of the difference Blend Mode by adjusting the Blend Range opacity ramps.
Lets take a look at a couple of examples.
These examples use the blue colored blend layer. In the above example the Blend Layer's grey channel Blend range was adjusted. The colors that are less in opacity than 195 will gradually have less opacity. In the above example the lower ramp up value was set to 72. Because there are a lot of colors in the underlying layer which have an opacity value less than 72, the Difference Blend Mode using these ramp up values has little effect.