Code placement anamoly

I have a movie clip on the main timeline called “map,” w/ tons of nested clips. On an actions layer on the Main Timeline I have the following:

map.nestedClip.onRollOver = function() {
this._alpha = 50;

the rollover event fires, but the alpha reduction does not. I replaced the alpha = 50, with a trace statement, and was successful, plus I get the hand when I roll over the movie clip, so I know that the event is registering. However, when I place the same code directly on the instance, it works fine. I thought the only difference between placing code on the timeline and directly on an instance was a matter of convention, and that timeline code is generally a better practice. Does anyone have an idea? thank you.