The nature of the game makes it very hard to maintain consistent and predictable difficulty.
Again, think of Candy Crush Saga - when new candies are generated, I can prevent them from forming groups with existing candies, thus increasing the difficulty. Or I can force them to form groups, making the game easier. But I can't do this all the time, or it will become too obvious.
Also, I can't predict player's moves. Player can make a clever or very lucky move, match lots and lots of candies and score lots of points.
I am gradually increasing difficulty as the player progresses through levels by introducing more obstacles/enemies and adding more tasks to complete. But the amount of points player can score in each level is still greatly depends on random generator...