How do you get...?

[The Barrow-Wight]

How do you get Bill from William, Chuck from Charles, Bob from Robert?

dictionary.com defines nickname as:

A descriptive name added to or replacing the actual name of a person, place, or thing.
A familiar or shortened form of a proper name.

I think the key words are 'familiar' and 'replace'.

Nicknames are replacements for real name that, though not always based upon the actual name, become famliar enough to stick.