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.