Changing surnames

I know that when you move two Sims in together their last names become the same (which is annoying, but that's a post for another time). And if you change the surname/last name of an adult Sim using the Ultimate Makeover, everyone in that household will have the new last name. But I've noticed that sometimes surnames *don't* change, either by asking one to move in with another or by using the Ultimate Makeover option. Right now I have a teen whose last name I can't change no matter what I do (moving her in with another Sim or changing her parent's last name). It's happened with adults in the past, too. I'm wondering if anyone else has noticed this or has any idea why this happens?