It could be that they are using a method like this. Generate a random number between 1 to 1000. If it is 777, give the reward. With a method like this,getting 1000 boxes doesn't guarantee the reward. Without looking at the code of the game, we could just speculate. Only the developer(of this version) or the person gave specifications to him can answer this question, forum staff and oasis people handling customer support could not answer this.