User blog:Neeeeeeenja7/Backpack SDU question? Please answer!

Ok. I am trying to get all the backpack SDU's for playthrough 1 (including DLC "Secret Armory" and "Zombie Island").

Well i am confused for this reason. I will go under my completed missions and look for all the claptrap rescue's since each one gives you a backpack SDU, and so i know which rescue's i haven't done yet. I have crimson fastness, scrapyard, krom's canyon, lockdown palace, the lost cave, new haven, old haven, the salt flats, safe house, tetanus warren, trash coast.

I have all of them done. And i know i still have the one for the "lumber yard" to do. But shouldn't i still have 45 slots and not 42 (which i currently have) since i did 11 rescue's and you start with 12 slots. And by my math 12 + (11x3) = 45. So why do i only have 42 slots?