EmberJS - Observe a computed property

Why the following codes are not equivalent?

With computed:

computed: Ember.computed('selected', function() {
    console.log('computed');
    return this.get('selected');
}),

observer1: Ember.observer('computed', function() {
    console.log('observer1');
}),

observer2: Ember.observer('selected', function() {
    console.log('observer2');
}),

With alias:

computed: Ember.computed.alias('selected'),

observer1: Ember.observer('computed', function() {
    console.log('observer1');
}),

observer2: Ember.observer('selected', function() {
    console.log('observer2');
}),

The first one only prints observer2 and the second one prints observer1 and observer2.