Actually, your code shouldn't be working for several reasons.
1) Unless you failed to give us the code for "mymovie"'s creation, then "mymovie" doesn't exist. So loadMovie is trying to load your .swf into a null target.
2) Even if the .swf loads (which from your original post, it does), then just logically speaking, setting an onload handler after
you load the movie doesn't make sense. As in every script, you need to set up the handlers before the events are fired.
3) I'm not 100% sure, but iirc you can't set an onload that way using the loadMovie function. If you want to set an onload handler, this is the correct way of doing it:
var mc_loader:MovieClipLoader = new MovieClipLoader();
var mc_listener:Object = new Object();
mc_listener.onLoadInit = function(target_mc)
target_mc._visible = false;