Wait, Jin isn't the one writing the journal entries, and if he is, what the hell is Kosuke telling our brother? But even if that statement isn't true, he did get access to better training via Mao sending him to Fang's sect. Jack himself said he took full advantage of it, while the MC is basically self-taught with some help.
Okay, this is actually fair, but even then. Diamonhead said that the best narration for Jin is him having 700 AP. There is no excuse for him and Rin outclassing 90% of the sect. Especially given how far behind they start, Rin doesn't even want to try.