Stuart, your fix doesn't make sense. If you calibrated the unit as per instructions, then cut a board using the saw's tape to set the fence then bent the Wixey attach bracket to match up with the size of the board, what you have really done is disturbed the Wixey calibration. The next time you calibrate it, it will be off. The readout doesn't care where it is in relation to the fence when you calibrate it, as long as it is firmly attached and the relative position of readout and fence doesn't change. You can actually calibrate it by inserting a spacer between the blade and fence if you don't want to scratch your fence, then use that same spacer between the rare earth magnet and the attachment point before hitting calibrate.
I am a long time Wixey TS digital fence user- over three years. I was one of the testers for Barry Wixey on the prototype over a year before the production unit was released and have been using a production unit continuously since then. I have had the same experience as DavidF- mine is dead on. It should not work the way you describe unless (check all these possibilities)- (1) you are not carefully locking your fence at the point where the blade just kisses the fence, before calibrating the unit, (2) you are pushing the fence against and actually deflecting the blade slightly when you calibrate, (3) you are trying to calibrate it when the blade is at a slight bevel, (4) you have a warped (wobbly) blade or arbor runout that gives improper calibration, (5) the magnet attachment point is not properly mounted to the fence and moves, (6) the rare earth magnet on the side of the readout unit does not make solid, flat, and firm contact with the fence attachment point, (7) the back on the Wixey readout unit is not properly tightened (4 little screws on the back) causing the sensor head too ride too loosely on the sensor strip - this is a common problem, and lastly but unlikely, ( 8 ) the unit is defective.