It's normal for your A/F indicator to jump around at idle and anywhere other then WOT. I am not surprised that it reads full rich on the X since just about all vehicles run rich from the factory.
Why have the indicator at all? To me it's meaningless unless you are adjusting your fuel trims with a piggy-back computer or a stand-alone.
Hell, the indicators you are probably using is junk anyway. You can't use it for any real tuning and IT'S ALL FOR LOOKS.
BTW, your volt meter might not have been fast enough to register fast voltage changes when the X isn't WOT.
It seems like you installed the gauge just fine. Relax and impress the chicks with your light show.