My really name is Zachary, shortened to Zac by every one except my mum and myself on legal or formal documents. As for why it is just Zac and not Zach I have always considered h to be part of the second syllable Zac-hary. Besides when you say Zac there isn't a ch sound like in the word catch at the end of it. Actually gained the name from the character Zachary in an old T.V. Show called Mr. Merlin as it was a good comprimise between unique and vaguelly biblical at the the time although now I doubt if either would matter to my parents.
I have had a few nicknames in my time as well my favourite being slush puppy which was used for a week in 2005 by fellow laser tag players at the nationals. It was due to my quick service of the slush puppy drinks and my seeming super human ablity at the time to make 25 of varying sizes in 5 minutes.