User blog comment:Zathronas/Speculative author: Family Tree part 2/@comment-10990964-20140908200649/@comment-3146930-20140908201947

We don't know what their father's last name is either. If Yang's mom and dad were married, Yang could have their last name if she changed it to his. However maybe because she left, he couldn't finalize the divorce and so he couldn't get remarried to Summer. As an apology, he allowed Ruby to share her name instead of his.

However there could be a lot of other reasons and we don't really know the naming system in the series. You would think to make things simple they would just use our naming system, but they could over complicate it if it is a plot point.