Well, the reason we capitalized "Light of the World" on some references and not in others has to do with a little thing called the article. When Jesus says, "I am the light of the world," he's giving a title to himself and that's capitalized. In the other reference, however, the article is not in the original language and it's more like he's the light for the world.

He's not the light of the world. He's serving as a light to the world. First time, the article is there in the Greek. The second time, the article is not there. So "I am the light of the world," capitalized. "I am serving as a light to the world," not capitalized.

So it has to do with what's in the original language.