[MX2k4]Assigning event functions

why does this work:
[AS]for(i=0;i<perPage;i++){
with(_root.attachMovie(“entry”,“entry”+i,i)){
_root[“entry”+i].onRollOver = function(){
trace(“poo”);
};
_x = 25;
_y = 25+(_height * i)+3;
}
}[/AS]
but not this:
[AS]for(i=0;i<perPage;i++){
with(_root.attachMovie(“entry”,“entry”+i,i)){
this.onRollOver = function(){
trace(“poo”);
};
_x = 25;
_y = 25+(_height * i)+3;
}
}[/AS]