Okay, here's my idea. When Garp is talking about leaving Luffy and Ace on the island, he mentions another name as well. I don't recall this name, whether it was male or female. So two possibilities. Their mother's surname, or the surname of the person Garp left them with is Portgas, and because Dragon is the most wanted man in the world, of course they'd have to hide the name Monkey. So why then is Luffy still "Monkey D. Luffy"? Think about his personality, especially when he was a kid. If he didn't want to change his surname, he wasn't going to change his surname! Ace seems more sensible and would have complied.
(you may wonder if Luffy would have been too young to make that decision, but there's the picture of Luffy when Garp is "training/being cruel" to him, and he's not incredibly young. And taht would have been before the point that Garp left them with the friend on the island. Plenty old enough to be stubborn about having to change his name.)
Correction: it was the mayor that mentioned the other person, who's name is Dadan.
