I have no true, determined reason to my knowledge. Honestly, I would have to believe it was developed when I was but a lad in the slow growing kingdom of Ostia. I was taught by others who lived in the same village as I to hunt. Hunting occurs very early morning, normally around 3 AM, so a keen eye is essential. Though I never became a truly great hunter, I could see better than any other I ever met. This includes fog.

While growing up, I slowly noticed my eyesight becoming more and more proficient, which helped me get into the military and train as a spy. I was trained to see through fog and darkness almost as well as in pure daylight. This is also the reason I generally awaken around 4 AM every day, so I can continue working on improving my sense of vision.

This may not be exactly the response you were looking for, but this is my best explanation.