Can anyone explain?

Open up a new document and in frame 1 of the main timeline post the following code:

function myFunction(){
trace(“stage height” + Stage.height);
trace(“stage width” + Stage.width);
}
setInterval(myFunction, 20);

the stage is set to default 550 x 400 but the output window shows changing values for height and width as the function is executed. The first returns values are 550 and 400 but thereafter they change.

Any ideas

Thanks