To be honest i dont know the exact way of how the cameras determine the level of rain, whether its rain on the windscreen or picking up rain in the surrounding area, but what i noticed when looking at the windscreen when its raining, the location of the camera is not ideal for detecting as the rain sits heavier on the windscreen the lower down you are, so where its so high, its above our eyeline and therefore less rain on the windscreen. Also take into account the curvature of the windscreen, the top middle is the least covered in rain as the rain also gets pushed to the side of the windscreen when moving at speed.
Not sure how other brands rain sensors are better in the same location as i believe they are all mounted there?