What causes damage variance?

What causes damage to vary when no other conditions change. Many times I have fought foes who almost kill me in one hit, so I heal them and stall until the foe uses a non aggressive move. Sometimes I will die from the move that I survived last time. What causes this variance? I don’t think it is random.

Pleaselog inorregisterto add a comment.

Ahem

Allow me to explain.This is the equation for damage calculation, quoted from Serebii

((((2Level / 5 + 2)AttackStatAttackPower / DefenseStat) / 50) + 2)STABWeakness/ResistanceRandomNumber / 100

Simple enough equation, just a bunch of variables to plug in. If you see that “Random Number” variable, this is what causes the variation in damage. The random number is generated between 85 and 100. Take the following situation:

Lv. 500 Magikarp uses Explosion on Lv. 500 arceus. Let’s call Magikarps attack 10,000, and arceus’s defense 8,000 (Just for kicks and giggles). We input the statistics:

And please, nerds, don’t come in and tell me that magikarp can’t learn explosion– take it from me

Ok, so this is how damage is calculated.

The damage dealt when a Pokémon uses a damaging move depends on itsAttack or Special Attack stat, the opponent’s corresponding Defense orSpecial Defense stat, and the move’s base damage. In addition, thevarious circumstances explained above will also affect the damagedealt. The damage formula is the following:

Level is the level of the attacking Pokémon.Attack and Defense are the working Attack and Defense stats of the attacking and defending Pokémon, respectively. If the attack is Special, the Special Attackand Special Defense stats are used instead. Some moves like Psystrikeuse other stat than usually move of this category would use (in caseof Psystrike, it uses Special Attack and Defense).Base is the base damage of the attack.

The only value that isn’t constant here is theModifier, so the random values have to come from there.Modifier is calculated as follows:

STAB is the same-type attack bonus. This is equal to 1.5 if the attackis of the same type as the user, and 1 if otherwise.Type is the type effectiveness. This can be either 0, 0.25, 0.5, 1, 2, or 4 dependingon the type of attack and the type of the defending Pokémon.Critical is 2 for a critical hit, 1 otherwise.other counts for things like held items, Abilities, field advantages, and whether the battle is a Double Battle or Triple Battle or not.rand is a random number from 0.85 to 1.00.The result is rounded down as remainders are not kept. It is possible to do zero damage1.

So in the damage calculation, there is always a number that is “irregular” causing the moves to do different amount of damage each time they are used.

Source:http://bulbapedia.bulbagarden.net/wiki/Damage_modification