Newbie question: getBytesLoaded() always returns 12

I’m pretty much a newbie when it comes to Actionscript, but bear with me please. How is it that when I use _root.getBytestLoaded() it returns the correct loaded size of the whole swf file.
But when I use MovieClip.getBytesLoaded() (or getBytesTotal()) on a movieclip (for instance one that contains just an image) it always returns the numbers 12. That’s strange.
For instance, I have a movieclip that has one image inside it.The movieclip’s instance name is hats. And when I use hats.getBytesLoaded() it always returns 12.