Fiber is actually a type of carb. But it’s a special kind of carb the body cannot digest, so it doesn’t impact blood sugar. That’s why fiber grams get deducted to calculate “net carbs,” because it adds itself into the carb count and then subtracts itself out of the carb count because it’s some kind of magical, weird, indecisive, sneaky little fiber carb.
Hence, when you’re adding adding fiber to a recipe, it cannot make other ingredients go negative-carb. Good try, though!