By checking the extra credit box, the raw unaltered value of points awarded are added to the parent category total.
You can choose to make a category extra credit by selecting the extra credit box on the category total for an entire category. this makes the value of the entire category become added to the parent category total.
7 + 2 + 100 = 109 --> 109/100 = 109%((7 + 2 + 2)/23) + 100 = 104.78 --> 104.78/100 = 104.78%