How are the timeout IDs allocated?

Could someone please explain me how are the IDs allocated? Or even better why it's not like I thought? :)

In this code below I would expect the ledTimer to get the same value (ID) over and over again but that doesn't happen.

var btnPin = BTN;
var ledPin = LED1;
var ledTimer = null;
digitalWrite(ledPin, 0);

var btnWatcher = setWatch(function () {
  if (ledTimer) {
    clearTimeout(ledTimer);
    ledTimer = null;
  }
  digitalWrite(ledPin, 1);
  console.log("before: " + ledTimer);
  ledTimer = setTimeout(function () {
    digitalWrite(ledPin, 0);
    ledTimer = null;
  }, 4000);
  console.log("after: " + ledTimer);
}, btnPin, {repeat: true, edge: "rising", debounce: 70});

Now when I press the button every 5 seconds I get what I expected:

before: null
after: 2
before: null
after: 2
...and so on

But when I press the button every 3 seconds then it's not what I expected:

before: null
after: 2
before: null
after: 4
...and +2 every time