That fraction is going to depend a lot on the definition and the reference. I believe the 97% is the US standard for how much of the natural caffeine in green beans must be removed. You will note how this can be manipulated by using a more caffeine-abundant variety. EU standards are more sensible, stated in terms of caffeine content in the final product.
Either way, commercial decaf processes and normal brewing methods will yield something like 5-10mg of caffeine in a "decaf" dose of coffee, which is an order of magnitude less than usual.
You're right that the caffeine isn't entirely removed, but it's supposedly more than 90% removed.
(I'm even seeing the number 97% mentioned a lot online.)
That fraction is going to depend a lot on the definition and the reference. I believe the 97% is the US standard for how much of the natural caffeine in green beans must be removed. You will note how this can be manipulated by using a more caffeine-abundant variety. EU standards are more sensible, stated in terms of caffeine content in the final product.
Either way, commercial decaf processes and normal brewing methods will yield something like 5-10mg of caffeine in a "decaf" dose of coffee, which is an order of magnitude less than usual.