2D billboards in Sketchup (trees, people, etc.) are not equally toned, or shaded (more exact). half of them, their "body"/ssurface in Sketchup 2018 (up to central turning axis) are PROPERLY under light, and the other half is SHADED. not in every position, but 60% of positions of the camera.
or...
this transition should be more soft,light transition to darker tones - to simulate "3D objects appearance" that are partly shaded by themselves alone...