Why was the damage different without any stats boosts?

Question

In a Showdown Battle:

Turn 1 -
Rayquaza used ExtremeSpeed!
The opposing Xerneas lost 52% of its health!

Turn 2 -
Rayquaza used ExtremeSpeed!
The opposing Gwyndolin lost 45% of its health!

Xerneas didn't raised any stats.
Why did the 1st move do 52% and the 2nd 45%?

Comments

Wait, did the opponent change Pokemon?

Answer 1

It's because when a move is used it does between a range of damage, for example:

252+ Atk Tough Claws Mega Charizard X Fire Punch vs. 252 HP / 0 Def Aegislash-Shield: 254-300 (78.3 - 92.5%) -- guaranteed 2HKO after Leftovers recovery

(254, 258, 260, 264, 266, 270, 272, 276, 278, 282, 284, 288, 290, 294, 296, 300)

↑ You see the numbers above? That's the range of damage the Mega Chaizard X in the calculation will inflict upon the Aegislash-Shield used in the calculation. (Aegislash's HP is maxed out at 364 for this calculation.)

Answer 2

To understand this, we need to look at the damage formula itself:

The one thing we need to focus on here is the Modifier part at the end, since all other factors are constant.

Here, we need to look at the very last point. Putting it simply, this part generates a random number between .85 and 1. And that will be the number used during damage calculation. This means an attack can change damage withouth any factor actually changing.