Check the bottom of
this post, and some further on. The additional weight springs aren't different spring rates, just taller springs (as shown in your specs). So when they have the extra weight on them, they compress to what a normal spring would without the extra weight. Hence without the extra weight these springs will add more lift than described. Also note that we're talking about GVWR +extra weight, meaning that this is when the vehicle is fully loaded plus the weight of the bumper/winch. If it's not fully loaded with bumper/winch then will get more than advertised lift. Unless you're loaded down for a week long overlanding trip you probably aren't near the GVWR +bumper/winch that would be required to get only the listed lift.