Several guesses:
1. Stuck shed you don't know that is still stuck.
2. Cataract
3. Genetically "dull" eye(probably not the case) :shrugs:
I work with a pine/bull snake which has a smaller/deformed eye compared to the other. It developed a cataract in the last 6 months. Sheds are always perfect. For definite verification, I'd see if a vet would examine it.