Accelerometer returned values...

I'm puzzled by what I'm seeing... I have a Puck programmed up to display the values coming back from the accelerometer :

require("puckjsv2-accel-movement").on();­
var idleTimeout;
Puck.on('accel',function(a) {
  LED.set();
  if (idleTimeout) clearTimeout(idleTimeout);
  else {
    // print("Motion", a);
    var s = "{'type': 'M', 'x':" +  a.acc.x + ", 'y':" + a.acc.y + ", 'z':" + a.acc.z + ", 'gx': " + a.gyro.x + ", 'gy':" + a.gyro.y + ", 'gz':" + a.gyro.z + "}";
    Bluetooth.println(s);
  }
  idleTimeout = setTimeout(function() {
    idleTimeout = undefined;
    LED.reset();
  },500);  
});

It certainly reports numbers, but they don't seem to bear much relationship to the magnitude of the movement. A small tap on the table produces : {'type': 'M', 'x':-644, 'y':133, 'z':14235, 'gx': -675, 'gy':-2386, 'gz':1043} and a major movement produces : {'type': 'M', 'x':2926, 'y':402, 'z':19069, 'gx': -675, 'gy':-2386, 'gz':1043}

what are the units?

TIA

Martin