Why is OutputStream write method printed before PrintWriter's println method?

I am using an OutputStream and a PrintWriter. Here's my code:

  OutputStream os = System.out;
  PrintWriter writer = new PrintWriter(os, false);

  writer.println("Hell");
  writer.println("Hello");
  writer.println("Hello");
  writer.println("Hello");
  writer.println("Hello");
  writer.println();
  os.write("45\n".getBytes(), 0, "45\n".getBytes().length);
  writer.println();
  writer.flush();
  os.flush();

The output is:

45
Hello
Hello
Hello
Hello
Hello

Why is 45 printed before Hello's, even though the PrintWriter is flushed before the OutputStream?

Edit: Please correct me If I am wrong about flushing.