Node.js archiver has unclear behavior

Question

I'm using the following code to zip an entire folder that consists of .txt files only and to send to the client as a .zip file.

var time = new Date().getTime();
var userId = parseInt(req.params.userId);
console.log("User ID: "+userId);
var output = fs.createWriteStream('user_archive/temp-'+userId+'.zip');
var arch = archiver('zip');

output.on('close', function () {
    console.log(arch.pointer() + ' total bytes\n\r__');

    res.header('Content-disposition', 'attachment; filename=archive-'+time+'.zip');
    res.header('Content-type', 'application/octet-stream');
    var filestream = fs.createReadStream('user_archive/temp-'+userId+'.zip');
    filestream.pipe(res);

});

arch.on('error', function(err){
    throw err;
});

arch.pipe(output);
arch.bulk([
    { expand: true, cwd: 'user_archive/temp-'+userId+'/', src: ['*.txt'], dest: 'messages-'+time}
]).finalize();

The problem I'm facing is that I am not receiving the same number of bytes when testing this code and console.log(arch.pointer() + ' total bytes\n\r__'); gives me values from 22 bytes, which correspond to an incorrect archive, to the correct number of bytes. Another thing to mention, after 5-6 consecutive tests I receive a correct archive until I stop and restart the node. Then it happens again: 5 incorrect archives followed by correct archives. My question: why this unclear behavior?