Just because Jackson is listed as the starter doesn't mean that he is going to get all of the carries, or even the bulk of them...
Spiller is still young, and he has to pay his dues... I know that it sounds stupid, but if you ask me... Jackson as the starter is just to keep the coaching staff from looking stupid if they were to start Spiller and him do something stupid...
Now if Spiller does great he gets more carries, but if he messes up they look like geniuses