Why when testing does Ember store.findRecord need a Ember.run while store.findAll doesn't?

Hope you can help me with the following:

I am writing tests for my serializer (using Pretender), following this guide: http://ift.tt/1opSXkW.

I am confused by the fact that i can call store.findAll in normal test code (no Ember.run needed), but when I call store.findRecord i need to put my code inside Ember.run for it to work - why is that ?

I am using: "ember-data": "^2.3.0", "ember": "~2.3.1",

test/unit/serializers/activity-tests.js:

import { moduleForModel, test } from 'ember-qunit';
import Pretender from 'pretender';
import Ember from 'ember';

var server;

moduleForModel('activity', 'Unit | Serializer | activity', {
  needs: ['serializer:activity'],
  afterEach() {
    if(server) {
      server.shutdown();
      server = undefined;
    }
  }
});

test('it serializes array responses', function(assert) {

  server = new Pretender(function() {
    this.get('/activities', function() {
      var response = [
        { "id": "1", "reference": "Tubby" },
        { "id": "2", "reference": "Spot" },
        { "id": "3", "reference": "Chestnut" }
      ];
      return [200, { "Content-Type": "application/json" }, JSON.stringify(response)];
    });
  });

  //
  // Why does this run without Ember.run loop 
  // (it also does when inside run)
  //   
  // Ember.run(() => {
    return this.store().findAll('activity').then((data) => {
      assert.equal(data.get('length'), 3);
    });
  // });
});

test('it gets a single record response', function(assert) {

  server = new Pretender(function() {
    this.get('/activities/:id', function(/*request*/) {
      var response = { "id": "1", "reference": "Tubby" };
      return [200, { "Content-Type": "application/json" }, JSON.stringify(response)];
    });
  });

  //
  // Why does this only succeed when inside the Ember.run loop 
  // while the findAll (above) can run without it ?
  //   
  Ember.run(() => {
    return this.store().findRecord('activity', "1").then((data) => {
      assert.ok(!!data);
      assert.equal(data.get('reference'), 'Tubby');
    });
  });
});

app/serializers/activity.js:

import DS from 'ember-data';

export default DS.JSONSerializer.extend({
});

app/models/activity.js:

import DS from 'ember-data';

export default DS.Model.extend({
  reference: DS.attr()
});