I have no idea either but just to add I've also known Sharon = Shaz, Shazza / Shazzer (in Bridget Jones), Darren = Daz, Marian = Maz (this was a woman I worked with), Karen = Kaz, Caroline = Caz. I think it's just because names which have an R in them aren't easy to abbreviate. Though why Brits do it and other nationalities don't, who knows?