I don't have an answer for you, but I'll throw in the observation that when you have your flash pointed directly (diffuser or not), the camera-to-subject distance is used in the flash exposure calculation, and the flash head zooms to the appropriate focal length (you can see the focal length on the flash's LCD). But, when you angle/rotate the head, the flash head no longer zooms. Not sure if that's a factor, but it is one thing that substantially changes between straight-on and bounce.