weird beforeunload event issue

I found a weird problem, with the beforeunload event.

Look at this code -

Client JavaScript:

var socket = io.connect('http://localhost:1337');

socket.on('connect', function() {
    window.addEventListener("beforeunload", function () {
        socket.emit('out');
    });
});

Server node.js:

/* Basics */
var express = require('express'),
    app = express(),
    server = require('http').createServer(app),
    io = require('socket.io').listen(server);

server.listen(1337, null);

io.set('transports', ['xhr-polling']);

// routing
app.get('/', function (req, res) {
    res.sendfile("index.html");
    app.use(express.static(__dirname));
});

io.sockets.on('connection', function (socket) {
    socket.on('out', function() {
        console.log('disconnected');
    });
});

If I go ahead and start the server and go to localhost:1337 and close the tab, the server not gonna log disconnected.

I figured out that if I delete the line - io.set('transports', ['xhr-polling']);
The server will log disconnected.

Why the server doesn't log disconnected in xhr-polling?

Thanks to everybody who tries to help.