Why does Cody Simpsons voice change when he sings?

So there's this girl who has this complete obsession with Cody Simpson on yahoo answers and she said his voice was deeper than Justin biebers at age 13.so I looked up an interview of Cody just to hear his voice and his voice is so much deeper than when he's singing.I mean I thought someone dubbed his interview but that's his voice,so why does his singing voice sound like a chipmunk compared to his real voice.to much autotune and can't really sing?