That's a bit of a misnomer: diets like Keto don't "allow" set amounts of carbs, they require you to find your own carb limit to stay in ketosis.

If you constantly blow out of ketosis, you'll literally accomplish nothing with the diet.

Hence why keto-stix and other indicators are important- no one value works for everyone.

I had to eat less than 25g/day to maintain the keto diet.