The rear springs are softer than the front. When you sit in the car they sink down more. If you have the car setup to sit level with nobody in it then it will sit lower in the back than the front once you sit in it (and it will sit really low and bottom out constantly in the rear if you put stuff in the hatch, kids in the back seat, and the wife in the passenger seat). That's why the back is higher.
Even if you don't ever plan to put more than yourself in the car the rear needs as much travel as it can get because when you accelerate out of a turn (not just when you launch from a stop) the rear squats down. If you're exiting a turn and you're near your adhesion limit and the car squats and bottoms out on the outside rear bumpstop it will upset the car and cause snap oversteer. 90% of the time when you see a powerful RWD car suddenly lose it and spin out exiting a turn this is why it happened. This is why the serious track guys don't run with their cars lowered excessively and if they lower them at all they run stiffer springs, upgraded swaybars, and progressive bumpstops.
