that's the beauty of it: the randomness is "bigger" for objects illuminated "just right" (half way through completely dark and 100% bright), but it still works for pictures taken in less than ideal conditions. And it's all because of the imperfections in all smartphone cameras. Out of 100.000 pictures taken in the exact same position with the same camera, do you think you can get two 100% identical ones?
Even if you could get the exact same conditions (illumination, position, etc.), there are always factors like that tiny little bit of degradation that happens each second you expose the sensor to light.
