Pictures of the octaweb show that the engines are not placed at the center of each octaweb compartment---they are closer to one side than to the other.
See the picture below for an example. Focus on just one section of the octaweb---any of the white panels at the top will do. The engine is much closer to the right side of the panel than to the left of it. You can trace the same pattern all the way around the octaweb.
Why? Is this just to make space for other components that must go in the same compartment? If so, then what might those components be?