Does he do it intentionally? Or is it just shoddy story telling? I don't know how many chapters he does this in, but the ones I noticed it in most were chapters 11 and 13. Sometimes he'll say "Sylvester says..." then sometimes its "I said." He shouldn't have thrown a narrator into the story halfway into the series if he wanted to use first person.
In chapter 1, the idea was that he was singing to his wife (I think initially it was going to be a one off song, and people kept asking 'what happens next?' and so he carried on). But as the wagon started rolling, he, for whatever reason, decided to make it in the third person. 'The Narrator' in chapter 1 is a version of the character (hiding in the closet), but the narrator in later chapters has disassociated himself from the character 'Sylvester'.
Basically it's a mess, he doesn't explain it, but who cares, there's far more awful things in this series to be laughing at than that!