You have a lot of noise in your atmosphere due to low detail (you can tell this because there is a "polygon"/low resolution look to the clouds). This is causing inaccurate results. You can try switching on Ray Trace Atmosphere to use a different rendering method that is higher quality, but then your quality depends on your antialiasing setting. Alternatively you can try increasing Detail, I would recommend 0.5 at a minimum, and then increase atmosphere samples until the noise goes away.
- Oshyan