Actually, it IS confusing. It's not straightforward at all. Nicknames make sense when they relate phonetically to their full names:
Sam for
Samuel,
Greg for
Gregory,
Mel for
Melanie, etc.
If we changed them to Same for
Samuel, Greyg for
Gregory, and Mail for
Melanie, they don't make sense one bit.
I would immediately think that I were pronouncing the person's name incorrectly