redirecting stdout goes wrong when writing stdout from multi-processes

main.js

var fork = require("child_process").fork;

var workers = []; 
for (var i=0; i<6; i++) { (function(i) {
  workers[i] = fork(__dirname + "/worker.js");
  workers[i].send(i);
})(i) }

worker.js

process.on("message", function(msg) {
  for (var i=1; i<=100; i++) {
    console.log(msg, i);
  } 
  process.stdout.end();
});

process.stdout.on("close", function() {
  process.exit();
});

$ node main.js > filename
$ wc filename
570 1140 2801 filename
(expects 600)

$ node main.js | wc
600 1200 2952

$ node main.js | cat > filename
$ wc filename
600 1200 2952