I've noticed a similar issue when I use flash in A mode. The camera sets the shutter speed depending on the focal length and does not change when I change the Aperture. For example, the camera sets a shutter speed of 1/80 for a particular focal length. I change the aperture without changing the focal length and nothing happens to the shutter speed - it's still at 1/80. It doesn't seem right to me. Has anyone else seem this before?