it happens. some just pick up on it more quickly; maybe that extra year made a difference, too. I know, in general, it's easier to pick up other languages when you're younger. a year shouldn't make a difference, but it might. my grandfather speaks English pretty well but not perfectly. he and his brothers came over from Italy at the same time, and his brothers speak English so much more clearly.
in dhiego's fight episode, he even mentioned that he was quickly assimilated. he was bragging about speaking fluently after 6 months; his only real standard for comparison must've been his brother.