The outsiders-why does Ponyboy keep saying he killed Bob?

In S.E Hinton's 'The Outsiders', Ponyboy doesn't keep saying he killed Bob, but mentions it only once when he has been arrested at the hospital where Dally had brought him after Johnny killed Bob to save Ponyboy when the Socs had jumped them. However, Darry, Johnny and Sodapop were there when the killing was carried out hence he was sure and correct about who murdered Bob and not him. Moreover, the killing was done in self-defense.