Confused about how spell accuracy is calculated

My wizard was using a scepter, and his inventory window showed that the accuracy of his primary hand is 40.

And then I let him cast Arcane Assault on a young wolf and saw the following information in the combat log:

Acc:47 - Ref.:49 = -2. Roll:26 - 2 = 24 (Graze). 7.6 = 7.6 Raw

I'm confused.

Arcane Assault itself grants +10 Accuracy, so isn't the number of Acc in the combat log supposed to be 50 (40 + 10)?