Back when I used to race those things I swear you wanted them as heavy as you could get them? I remember every year this same hillbilly would fill his overalls with weights then throw them in his sons car at the top of the hill after weigh in...?
You wanted them up to max weight but you can add weight to get to the max weight. You wanted to keep the weight as low as possible and you wanted to make the car as stiff as possible. The carbon fiber made the car stiffer and kept weight off the top of the car allowing you to put more weight low in the car.