[She notes the different last name and assumes that either Luffy or Ace are using their mother's maiden name, it makes sense with both standing as pirates. Using the same name would drag attention to it and either give one of them enemies that try to get them in order to hurt the other, or make them live under the other's shadow as "Luffy's brother" or "Ace's brother" rather than as pirates on their own.

She's more interested in the nickname anyway.]

Would you prefer me to refer to you with something else instead of Ace?

[Just in case he doesn't really like to be called by his name by strangers, even if he didn't complain before.]

... "Fire Fist", is that just a nickname or do you possess powers like Luffy?