Different universe. Kirk isn't exactly identical over there and is now more rebel, without a Father to steadily influence him up through the ranks. He's now the late bloomer who achieved early. Quickest rise to the Captaincy, in the history of anything.
Perhaps Garth will be a different kind of mad man too? Different backstory that causes his madness.
It's probably like comparing Baltars. Campy 70s John Colicos, or slightly more complex 00s James Callis.