Why does the amount of caffeine in decaf coffee say a range of 3-15 mg of caffeine and not just one number?

Question

So does that mean every cup varies with how much caffeine it has?

Answer 1

Because the process of de-caffeinating coffee isn't an exact science.
Decaf just means reduced caffeine .. not no caffeine.
There is no set amount that the coffee has to be decaffeinated to.

Answer 2

it means different ways of brewing the coffee effects the amount of caffeine in it.
Thats the same for regular coffee too.

Answer 3

Yes, that is what it means.

Because the process they use to decaffeinate it isn't ever going to be refined to anything more exact than that.

Answer 4

Because different coffees have different amounts. And everyone makes coffee differently - the amount of coffee used varies, the amount of water used varies.