How are the new ACT essays graded?

I took the October ACT and just got back my essay grade. I got a 12, 11, 11, 11 on the individual categories of the writing section. However, I only got a 31 (out of 36) on the writing section. I think it is a bit low considering my grade in the individual categories. If my it was proportionally calculated I would have gotten a 34 (45/48*36 = 33.75 ~ 34), which is clearly much better than a 31. Does anyone know how they calculated grades for the writing section?