Is it true that you can subtract the fiber grams from the total carb grams for each food? If so, where and how did this come about? Is there any science behind this or only theory?

In the book...(and you HAVE read it, right?) Dr A explains that since fiber is not digested and passes through the system unchanged, it doesn't affect blood sugar or insulin levels, thus isn't included in your net (or impact) carbs.

OK, so we don't count it. But, why is it subtracted from the carbohydrate total? Does that mean that fiber is actually a carbohydrate? So if a food has 7 grams of carbohydates and 4 grams of fiber, 4 of the total carbohydrate grams are actually fiber and do not count towards your carbohydrate total?

Basically...If you have a food with 7g of Total Carbohydrates and 4g of those are from Fiber, you only count that food as having 3g of Net Carbohydrates. I.E. it would count as 3g towards your 20, 30, 40, whatever gram limit. This is why most Green veggies are very low in Net Carbs. Some of the really dark-green ones get as much as 75% of their carbs from Fiber. Fiber also has the added advantage of lowering the glycemic index (insulin/blood sugar spike) of the food it is in. This is one reason that Green Vegetables which derive 50-75% of their carbs from Fiber are very low glycemic, while potatoes which derive a meager 7-10% of their carbs from Fiber, are much higher glycemic.