Nicknames don`t have to be exactly shortened versions of the name. Have a little leeway. Sally and Bess don`t sound much like Elizabeth and Sarah but they were originally short forms. Harry and Hank are short for Henry too...I could go on. There`s a lot of evolution in nicknames, for example Johan became John which became Hankin which gave Hank, also used for Harry, but which also gave Jankin, in turn Jackin which became Jack. Quite a history. Besides, sometimes parents like a short form but not the "legit" root name. My brother`s name is Roderick but we call him Rory, even though Rory is an independent name and not a derivative of Roderick.