It's called a high roll, not a high role. In all games, when using moves that call the damage formula (i.e. not moves like Dragon Rage or Seismic Toss), there is an intentional random source of fluctuation designed to make it so that you can't simply rely on the second hit dealing exactly as much damage as the first one did, and in borderline cases where the first hit did very close to 50%, gives you something to consider as far as whether you want to risk going directly for the KO next time.
In general, the random fluctuation is designed to subtract up to about 15% off the move's idealized damage figure. In the GB games (up through Crystal), this was accomplished by (in effect) rolling a 39-sided die, numbered 0-38, and subtracting the rolled number out of 255 as a fraction of the damage. From the GBA era onward, the metaphorical die is reduced to having 16 sides (0-15), and the die result is taken as a fraction out of 100 to be removed.