Code question

in the first frame of a movieclip i have placed the following code to simulate a button. [AS]
this.onRollOver=function(){
trace(“onRollOver”);
gotoAndStop(2);
}
this.onRollOut=function(){
gotoAndStop(1);
}
this.onPress=function(){
gotoAndStop(3);
}
this.onRelease=function(){
gotoAndStop(2);
}
[/AS]
and on the instance of the movieclip i placed the following code


on(rollOver){
	trace("rollover");
}

and it traces
rollover
onRollOver
which is great and exactly what i had hoped it would do, but i did not expect it as i thought the on(rollOver) overrode the onRollOver function handler in the first frame?

or are these 2 seperate events that get fired and i could just as well go


this.rollover=function(){

};

which does not work. what is going on here?