Who watched the anime and read the manga closely, can see a difference in the order of the events in the first episode of the anime to the first chapter in the manga.
In the manga, once Light has found the Death Note, he takes it home and laughing about that it must be a fake and some kind of joke, and then, the story jumps a couple of days ahead when Light is meeting Ryuk and then explains his testing of the Death Note in the couple of days that has been past and his ideology of using so.
In the anime, however, as soon as Light thinking it a joke he tests it for the first time, frightening by his action, goes to test it again, and then we jump couple of days later for the first meeting of Light and Ruyk, explains his ideology there.
Although it can be said, that the major change from the manga is due the commercial break featuring the How To Use screen, and it couldn't be done after the meeting with Ryuk, there's actually a reasonable explanation to the anime change even without that excuse - the anime used Light's ideology to create utopia as the climax of the episode, presenting it as the main event to the episode to open the anime, while in the manga his ideology is just a continue to his testing and only declaration of him as a god is were the climax at the chapter, so the question is going backward - why the manga had designed in that order and not the other way around, like the anime, presenting Light's ideas as the main event in the episode.
