How to recalibrate/reset Magnetometer?

I noticed when testing similar code that the first reading after offsetting with zero = Puck.mag(); gives x, y and z values close to zero, as you'd expect, but second and subsequent jump to values that vary (+/-) quite a bit from zero, and actually found it better to do use a later reading as a zero/offset, making a couple of calibration passes as below

var offset = {x:0, y:0, z:0};
var calPass = 2;

function readMag(mag){
  if (calPass > 0) {
    // calibration pass
    offset.x = mag.x;
    offset.y = mag.y;
    offset.z = mag.z;
    calPass--;
  } else { 
    // provide reading
    mag.x -= offset.x;
    mag.y -= offset.y;
    mag.z -= offset.z;
    console.log(mag);
  }
}

Puck.on('mag', readMag);
Puck.magOn();

I doubt it matters much, as slight changes in orientation of Puck still move values considerably, regardless, but it seems the Puck.mag() reading doesn't quite do the zeroing/reset I expected.