It's because, after all of the math is layed out to calculate damage, it'll be a "Variance", which is an RPG term used to define the range of damage that an attack can do.

Let's say that Staraptor has an attack stat of 153, and opponent has a Defence stat of 94 and 128 HP. Now let's assume that, this time, the attack does about 92 damage to the opponent's HP. Variance makes sure that Aerial Ace won't ALWAYS deal 92 damage. Next time, it may do more, like say, 104 damage, or less, like say, 85 damage.

It's all in the Math of calculating damage, Staraptor. =)