Issue
Switching the "Ground shadows" on and off can lead to different lighting conditions on the ground surface and sometimes produce results as if the shadows are always active.
Cause
The "Ground Shadow" option doesn't act like a cast shadow on/off switch. When "Ground Shadows" is Off, the VRaySun is shadowed or not everywhere, depending on whether the Camera shadowed.
In the following examples, the camera gradually gets shadowed. Notice that the scene darkens with either "Ground Shadows" on or off.
The render results depend on the Clouds parameters setup for height, density, offset, etc., and the Camera and VRaySun's position. Not only tweaking the VRaySun's parameters, but the Camera and VRaySun positions affect the scene's lighting.
Read more about the VRaySun's Cloud parameters in the Documentation: