Why is the surname Huo?

The surname is Huo because his father's real name is Huo. Huo Qingtong's father MuZhuoLun is just a pen name written by Jin Yong in the book, and his real name is Huo. Due to the conflict with real history, Huo Qingtong's father was changed to Muzhuolun. So the surname is Huo because his father's real name is Huo.