remove callback bug in backbone?

backbone-events

'use strict';

var chai = require('chai'),
    expect = chai.expect,
    sinon = require('sinon'),
    sinonChai = require('sinon-chai');

var Backbone = require('backbone');
var _ = require('lodash/dist/lodash.underscore');

chai.use(sinonChai);

describe('Backbone.Events', function() {
  var myObj;

  beforeEach(function() {
    myObj = {};
    _.extend(myObj, Backbone.Events);
  });

  it('can remove custom events from objects', function() {
    var spy1 = sinon.spy();
    var spy2 = sinon.spy();
    var spy3 = sinon.spy();

    myObj.on('foo', spy1);
    myObj.on('bar', spy1);
    myObj.on('foo', spy2);
    myObj.on('foo', spy3);

    // unbind a single callback for the event
    myObj.off('foo', spy1);
    myObj.trigger('foo');
    expect(spy1).to.have.callCount(0);
    expect(spy2).to.have.callCount(1);
    expect(spy3).to.have.callCount(1);

    // remove all 'foo' callbacks
    myObj.off('foo');
    myObj.trigger('foo');
    expect(spy1).to.have.callCount(0);
    expect(spy2).to.have.callCount(0); // spy called once, why??
    expect(spy3).to.have.callCount(0); // spy called once, why??
  });
});

How come spy2and spy3 are still around even after unbinding them from foo event?