When using Aperture Priority mode and you change the aperture, the shutter speed must change to keep the same exposure (keeping ISO constant in this case). For example, if f/8 and 1/125th gives you the correct exposure and you change to f/11, then the shutter speed will be 1/60th to give you the same (correct) exposure. When using Exposure Compensation, adding +1 will cause the camera to "overexposure" one stop from what the camera thought should be the "correct" exposure. For example, if f/8 and 1/125th is the "correct" exposure and you then select +1 Exp. Comp, the result will be f/8 and 1/60th.