I'll tell you some more about that what that Marjolein girl has been trying to tell you.
Look at me, for example. I was born Lucia Gottfrida Maria [surname], and I'm called Lucille in every day life. Lucia Gottfrida Maria are actually my Christian names (or baptisal names; I was baptized a Roman Catholic), which also appear on my birth certificate, passports, etc.
It's kind of a tradition for everyone in the Netherlands to have Christian names (mostly 3 or 4), whether you're Catholic, protestant or reformed, and to get called by your parents and close friends something other than your Christian names (depends on the parents' choice, actually).
It's like Americans do with their children. They can give a boy AlexanderJames [surname] as his official name, but if they choose to call him Alex in everyday life, then he'll be an Alex for all his life (it's anyway kind of a tradition in the US to shorten full names and to call someone by their nickname for all their life), as his relatives and friends will follow his parents' example.
It's like that here, too, but the 'nickname' is more official here. When a Dutch parent fills in a form of application for when he wants to enroll his child in a school, the form will always ask what the child's name in every day life is, so that the school will call this child this name, too. It'll often appear this way:
SURNAME: Janssen FULL NAMES: Petrus Johannes Theodorus CALLING NAME (name in everyday life): Pieter BIRTH DATE: February 28, 1999. BIRTH PLACE: Amsterdam.
And so on.
I hope this kind of gives you a better understanding of the common Dutch naming system. :-)