[F8] Why does the variable lose definition?

[a_modified_dog]

or break it down even further to check the logic -

downM.onRelease = function() {
varInc = _root.i++;  trace(varInc);
_root.goalY = _root.myYArray[varInc];  trace(_root.goalY); // 
slideY(_root.goalY);
};

function slideY(num) {
trace(num);
_root.dataFile.onEnterFrame = function() {
_root.dataFile.diff = Math.abs(_root.dataFile._y - num);
trace(_root.dataFile.diff);
....
}