A question about names

I need a little help here. There's something I just don't understand. My name is Matthew. My parents called me Matt. My teachers called me Matt, except for my kindergarten teacher who called me Matthews, but that's a much longer story. When I write my name, I write Matthew. Yet people still call me Matt. I wrote Matthew on my job application, yet somehow, the name placard on my cube says Matt. I'm not complaining mind you, Matt is a fine name. It's a little funny when it's written next to Burns (think wrestling, trust me, it's hilarious) but over all I'm happy with it. What I don't understand is this. If my parents were going to call me Matt, why didn't they just name me Matt?

My wife's name is Ginny. We call her Ginny. On her birth certificate, it says Ginny. Not Ginevra, Virginia, Ginevra, Geneva or Genevieve, Just Ginny. My son's name is Garrick. We call him Garrick. My other son's name is Riley, we call him Riley, (sometimes I call him butt head but mostly it's Riley.) I have a friend from college, his name was Bobby, ok, we called him Bob, but his momma called him Bobby. And then there's Jimmy Carter. His name isn't James, it really is Jimmy.

So my question is this, Why do parents name their children something that they have no intention of calling them? (except when they're in trouble but middle names work nicely for that as well) I'm not criticizing, I really don't understand. Can anyone help me out?