User blog comment:Derrmann/By Jove, I've got it! (Totally know why Ruby and Yang have different last names)/@comment-24138591-20131116210626

Monty Oum even said they were blood related so no this is nto why they have different last names, and it will probably be told in the next episodes.

Here's a list of this kind of stuff

http://rwby.wikia.com/wiki/User_blog:Darkness_Inferno/The_RWBY_AMA_on_Monty%27s_twitter