Ah ;D Makes sense!
I googled it though, because I was curious. According to wikipedia (source
"Catherine shares ancestors with her husband Prince William; the closest relationship is via William's mother through a common descent from Sir Thomas Fairfax and his wife Agnes Gascoine, making the couple fifteenth cousins. She is also related more distantly to William through her father's side through common descent from Sir William Gascoine and his wife, née Lady Margaret Percy. The couple are also possibly twelfth cousins once removed, with circumstantial evidence suggesting that they are both descended from Sir Thomas Leighton and Elizabeth Knollys."
I just thought it was interesting ;D