Why are people named William called Bill, Robert called Bob,etc?

Answer 1

Apparently (according to my mother-therefore it is correct *rolls eyes*), it's because the Brits like to rhyme names. William gets shortened to Will, which then becomes Bill.
Robert, Rob, Bob.
Richard, Rick, D*ck.
etc.

This is a good site, if you are curious about nicknames:
http://www.geocities.com/edgarbook/names/other/nicknames.html

Answer 2

I wonder that to