issue with collection.update callback params

app.js

/**
 update stats for the URL
 */
hit_emitter.on('newHit', function(url, hit){
    var data = { "$push": { "stats.hits": hit }, "$inc": { "stats.hitcounter": 1 } }       
    urlProvider.update(url, data, function(error, url){
        console.log('updated hit: ' + JSON.stringify(url));
    });
});

console.txt

null
{ '$push': 
   { 'stats.hits': 
      { created_on: Tue, 19 Oct 2010 18:12:58 GMT
      , referer: 'http://localhost:3000/'
      }
   }
, '$inc': { 'stats.hitcounter': 1 }
, modified_on: Tue, 19 Oct 2010 18:12:58 GMT
}
updated hit: {"$push":{"stats.hits":{"created_on":"2010-10-19T18:12:58.684Z","referer":"http://localhost:3000/"}},"$inc":{"stats.hitcounter":1},"modified_on":"2010-10-19T18:12:58.686Z"}

provider.js

UrlProvider.prototype.update = function(url, data, callback) {
    this.getCollection(function(error, urls_collection) {
        if( error ) {
            callback(error);
        } else {
            data.modified_on = new Date();
            console.log('updating url with ' + sys.inspect(data));
            urls_collection.update( 
                { _id:url._id },
                data,
                function(error, url) {
                    console.log(sys.inspect(error));
                    console.log(sys.inspect(url)); // url is actually == data
                    if (error) {
                        console.log(error.message)
                        callback(error)
                    } else {
                        callback(null, url);
                    }
            });
        }
    });
};