The draw length generally doesnt make the 31 - 34 inch bow less accurate.

What makes the 31" bow less accurate is the fact that it is less stable. ie- an object at rest will remain at rest unless acted upon by an outside force. The shorter bow is easier to torque and cant because it is a smaller/lighter object and therefore easier to enact that outside force. (your hand, follow through)

And I will agree with Rack about the brace height not being so much to do with the power stroke, but the closer it is to the rest. I beleive when the arrow is being propelled by the string, it is far harder to "cushion" itself against the rest than if it was off the string beforehand.