The truth of the matter is, they are not.
If you look at chia for example: a serving of 25 grams of chia has 10g of carbohydrates and 10 grams of fiber.
People trying to avoid carbs in their diet would obviously say this is way too much and I am not going to eat chia.
The issue here is that fiber is counted as carbohydrates in any analysis done on a food. So in this case the reality of the situation is that chia has no carbs that you want to avoid. Why is this so? Because fiber is not broken down into glucose.