I think what's confusing you is the idea of bread being 100% carb. What they mean is 100% calories from carbs. Bread (and everything) will contain other non-caloric things that have weight. For example, bread has moisture that adds weight without calories. Even pure protein powder that is carb-free and fat-free won't have as many grams of protein as it has grams of weight because it contains things like ash that contribute weight without calories.

Hope this helps.

~Mel