The FBW auto brake settings are predetermined deceleration rates and therefore the braking system modulates the pressure to achieve the requested rate when autobrake is used. With Low selected the reverse thrust on anything higher than idle can result in no actulal braking on the wheels. This is no real problem apart from cost, since with carbon brakes its the number of apllications rather than the time and pressure exerted that cause wear. So idealy low reverse and low auto brake will be used together.