Physics Learned Wrong!?

programming flash
1

Hey in school I always learned this


                    +y 
                     |
                     | 
                     |
       -x <------------------ +x
                     |
                     | 
                     |
                    -y

so can someone please explain to me why in flash the Y is flipped?

I have a code which is very simple moved my movieclip left, right, up, down with arrow keys.

here is the actual movement part


// move every frame
function moveMascot(event:Event) {
	var speed:Number = 5;
	
	if (leftArrow) {
		mascot.x -= speed;
	}
	if (rightArrow) {
		mascot.x += speed;
	}
	if (upArrow) {
		mascot.y -= speed;
	}
	if (downArrow) {
		mascot.y += speed;
	}

now shouldn’t uparrow be += and downarrow be -=??

Thanks