What is the cause of minor fluctuations in damage output of an attack?

So, recently I’ve been trying to get past the Totem Kommo-o in Ultra Moon. Now my strategy has kind of varied with each failed attempt, but it starts with my Mr. Mime using Shattered Psyche. Now, since Mr. Mime is part Fairy, the Kommo-o will always lead with Poison Jab, and that’s where things get odd.

As a side note, this isn’t the only time I’ve observed this effect happening, but this was the most recent example and the only one I can remember, so I’m using it for my question.

Pleaselog inorregisterto add a comment.

random is a random factor between 0.85 and 1.00 (inclusive):From Generation III onward, it is a random integer percentage between 0.85 and 1.00 (inclusive)In Generations I and II, it is realized as a multiplication by a random uniformly distributed integer between 217 and 255 (inclusive), followed by an integer division by 255

–Bulbapedia

It is this random variable that causes the fluctuation, and there’s no humanly way to control the output. This why battle simulators use an RNG (Random Number Generator).

This is why we see

Possibledamage amounts: (231, 235, 238, 242, 243, 246, 250, 251, 255, 257, 259, 263, 265, 269, 270, 274)

in thePS! Damage Calculator.