Movieclip _rotation causing off-center rotation

How do you determine at what point a movieclip will rotate when rotated via actionscript’s _rotation property? I thought it was through the pivot point (which you can adjust with the free transform tool). I have that sucker dead center and the movieclip still rotates just a bit off - well, by a bit I mean that by the time it hits 180 deg, its x and y are around 5 units off.

Here’s the code I am using:
onClipEvent(load){
this.roundCircleyButtonBtn.onPress = function() {
_root.navigationMC.dialmainMC._rotation = 40;
}
}