Data type changes at compile time???

[mark goldin]

I have the following code:
public function result(data:Object):void
{
     // Check data.result
     trace(data.result[0]); I see data type Object
     // but if I add this code:
     trace(data.result[0] as someVO); then at the first trace I see data
type as of someVO
}

How can run time know that I am about to cast data?

Thanks