The points possible settings can be confusing because there are actually two values that can be set. As shown below, you can set both the relative weight in category and the grading scale.
Relative weight in category will determine the weighting of the item within its category. The higher the value you set, the heavier the weighting will be on this item within the category to which it belongs.
Grading scale is your actual points possible for the item.
Generally, is it simplest to set the relative weight in category and the grading scale equal to one another. However, there may be times when you desire to set them independently. For example, I can choose to score an assignment out of 10 for my convenience, but may want a heavy grading emphasis on the assignment. So, I set the grading scale at 10, but the relative weight in category at 50 (or higher).