SetWatch can miss interrupts?

Yesss, this seems to work fine:

function onInit () {
  setDeepSleep(1);
  setBusyIndicator(LED1);
  setSleepIndicator(LED3);
/*
  var t;
  setInterval(function () {
    t= E.getTemperature();
  }, 3000);
*/
  setWatch(function (a) {
    digitalWrite(LED2, digitalRead(BTN));
  }, BTN, {repeat:true, debounce:0, edge:'both'});
}

I don't understand why does the red led stay on until the first BTN press?