Difference between this.instanceName and this.getChildByName(instanceName)

Hi,

i’m a little new to Flash and I haven’t managed to figure this out yet:
What is the difference between dragging a movieClip to stage from the user interface and inserting it from code? What I mean is…I noticed that if I drag a movie clip from the library to the stage, give it an instance name (‘xx’), I can access it from code using “this.xx”. However, if I add the movieClip from code using:

var ch = new MovieClip();
ch.name = ‘xx’;
addChild(ch);

I cannot access it using this.xx, but I have to access it using this.getChildByName(‘xx’);

Why is that? Shouldn’t the 2 processes(dragging and adding by code) do pretty much the same thing?
10x in advance and sorry if the question might sound silly.
Cheers