Fraternal twins share different family names

It's said these things happen in many families, but when first heard this it felt really strange. Here they are- Fang Yunhan, the little brother and Wang Shunqin, the big sister. We happened to meet them while we were waiting for bus at road side. They don't seem alike very much, do they? But they were on the same dress and of the same height, so we went up to them and asked boldly, "Hey, are you little guys twins?" And then, hah, they said "Yes, we are"! How fortunate we were! We were lucky enough to meet twins everywhere we went! 

As they were busy going on their way, we didn't talk much. They wrote down their names on our notebook and only then that we found them share different family names. In fact, their names don't rhyme at all. Sorry that we didn't have enough time to ask them why, but maybe, they each follow their father's and mother's family names respectively like many other families with twins?(Couples don't share the same family name in China.) Is it true, you little guys?