Puck.light values depend on how often it's called

The subject says it all - I get different light sensor readings depending on how frequently I call Puck.light().

These are measured in constant lighting conditions:

Average light at 0.5 Hz: 0.60877327967
Average light at 1 Hz: 0.61237373737
Average light at 10 Hz: 0.34283262310
Average light at 25 Hz: 0.39304174558
Average light at 100 Hz: 0.46682646780
Average light at 1000 Hz: 0.50218986742

The values seem to be roughly:

• at a maximum when below 1 Hz,
  
• at a minimum at ~10 Hz,
  
• rising (up to some limit) when above 10 Hz.
  
  

Using the following code:

ITER=10;
var experiment = function(dly) {
  var arr = [];
  var f = function(c) {
    arr[c] = Puck.light();
    if (c<ITER) setTimeout(f,dly,c+1);
  };
  setTimeout(()=>console.log("Average light at " + Math.round(100000/dly)/100+ " Hz: "+E.sum(arr)/arr.length),(ITER)*dly+50);
  f(0);
};

experiment(100); // set the argument to number of milliseconds between readings

Any ideas why this is happening?