Ah, the controversial Starkov/Starkova question. The primary reason was that I just didn’t like the sound of Starkova (or Morozov), so I decided that in Ravka, surnames wouldn’t be strictly gendered.
I talk a little bit more about language here and here.
In the interest of full disclosure, I’m also going to point you to this post by Rose Lemberg. She takes me to task in no uncertain terms, but the post is interesting and the comments are really thoughtful.