The boy called Bill (but named Karl)

My dad's name is Bill.  Bill Smith.  Only it's not.  He is called Bill, as in short for Karl Knight Smith.  Got it?  Makes perfect sense doesn't it?
Well, if you don't QUITE follow, let's back up a bit...
When little Bill was born his mother Roberta and father Knight agreed, that being he was the first born child, Knight would have the honor of naming him.  And so Knight, being the ever practical ranching farmer, did name him - named him Bill.  Bill Smith.  Just about the most common name possible in the English speaking world.  Easy to say when you need to.  Not a lot of syllables.
Little Bill's mother was not so found of this decision.
"What????  You can't name him Bill!  Bill Smith!  What kind of person would name their son that?  We'll name him Karl.  Karl Knight Smith.  Karl after my grandfather and Knight for his father," Roberta was sanctimonious in her decision.
So was father Knight.
"Fine," the proud father announced.  "Name him whatever you want, but I'm calling him Bill."