Board Thread:General Discussion/@comment-26955524-20160606205445/@comment-3146930-20160606210535

It's not uncommon for a child to take on the mother's last name if the mother herself doesn't want to change it. Or maybe they weren't married at the time and Ruby kept her mother's name just in case the relationship went south in the future. I mean we know Summer didn't change her last name based on her gravestone, so after Ruby was born they probably decided to keep the last name for their own personal reasons. Either way until we get that eventually Summer Rose flashback episode, we won't know the specifics.