I think it's possible that Zack inherited the attitude but not the skills to back it up. Or that Zack (as younger sons sometimes do) faced less scrutiny growing up, and so was more self-confident than his older brother.

Those scenes in Act of Contrition implied to me that Zak was unsure that he deserved his pass mark, and so was calling Kara on it. Which makes me think that he didn't have the attitude.