Knockout : context changed before the instruction in observable.subscribe

I'm trying to separate my Model from my ViewModel since I need the Model in two different ViewModel. So i've tried to this :

var ConversationManager = {
    conversations: ko.observableArray([
        {id: 3, receiver:'Toto'}
    ])
};

function ConversationDialogViewModel(){
    var self = this;

    ConversationManager.conversations.subscribe(function(context){
        console.log('Before :', context); // context.test == true ???

        for(var i = 0; i < context.length; i++){
            if(context[i].id % 2 == 0 && !context[i].test){
                console.log('Insertion Called');
                context[i].test = true;
            }
        }
        console.log('After :', context);
    });

    ConversationManager.conversations.push({id: 4, receiver:'Jean'});
    ConversationManager.conversations.push({id: 5, receiver:'Paul'});
    ConversationManager.conversations.push({id: 6, receiver:'Bleu'});
}

Somehow, in the first console.log, when i push "Jean", in the browser, the variable "test" exists and is correctly set to true even before i actually tell JS to do it. How is that possible ? And, am I correctly separating my concerns ?