Fetching Models to a Collection

I am pretty new to Backbone and just came across this confusing issue. I am trying to fetch models in to my collection in a Express node.js server with the following code :

app.get('/tweet', function(req,res){
    res.send([{ name: 'random_name' }, {name: 'diren_gezi'}] );

});

and my backbone code looks like this:

var PostsApp =  new (Backbone.View.extend({
    Collections: {},
    Models: {},
    Views: {},
}

start: function(){
    var data = {
        posts: [
        {name:"gorkem"},
        {name: "janish"},
        {name: "akash"}
    ]
    };
    var posts = new PostsApp.Collections.Posts(data.posts);
    var postsView = new PostsApp.Views.Posts({collection: posts});
    posts.url = "/tweet";
    posts.fetch();
    console.log(posts.length);
    console.log(posts);

}
}))({el : document.body});

I would expect console.log(posts.length) to return 5, because I am adding 3 when I initialize and 2 more when I fetch. Or even if the fetch method erases the collection and re-populates, I would expect posts.length to return 2. However it returns 3, but when I look into the post object from the console I only see two models, the ones coming from the fetch() method. What is the reason fror this ?