Auto sort on a grid - doesn't update after refresh of data?

package j3qsost7$v3;
public class MyMail {
 
  private String subject;
  private String sender;
  private Integer size;
  public MyMail(String subject, String sender, Integer size) {
    this.subject = subject;
    this.sender = sender;
    this.size = size;
  }
  public String getSubject() {
    return subject;
  }
  public void setSubject(String subject) {
    this.subject = subject;
  }
  public String getSender() {
    return sender;
  }
  public void setSender(String sender) {
    this.sender = sender;
  }
  public Integer getSize() {
    return size;
  }
  public void setSize(Integer size) {
    this.size = size;
  }
}

package j3qsost7$v3;
import org.zkoss.zk.ui.*;
import org.zkoss.zk.ui.event.*;
import org.zkoss.zk.ui.util.*;
import org.zkoss.zk.ui.ext.*;
import org.zkoss.zk.au.*;
import org.zkoss.zk.au.out.*;
import org.zkoss.zul.*;
 
public class MyRowRenderer implements RowRenderer {
    public void render(final Row row, final java.lang.Object data) {
        MyMail mail = (MyMail) data;
        new Label(mail.getSubject()).setParent(row);
        new Label(mail.getSender()).setParent(row);
        new Label(mail.getSize().toString()).setParent(row);
    }
}

package j3qsost7$v3;
import org.zkoss.zk.ui.*;
import org.zkoss.zk.ui.event.*;
import org.zkoss.zk.ui.util.*;
import org.zkoss.zk.ui.ext.*;
import org.zkoss.zk.au.*;
import org.zkoss.zk.au.out.*;
import org.zkoss.zul.*;
import java.util.*;
public class TestComposer extends GenericForwardComposer{
  private Grid inboxGrid;
  public void doAfterCompose(Component comp) throws Exception {
    super.doAfterCompose(comp);
 
    inboxGrid.setModel(new ListModelList(getData()));
  }
 
  public void onClick$refreshBtn(Event e){
    inboxGrid.setModel(new ListModelList(getUpdatedData()));
  }
  /* simply return a small model here , you could read data from database for your own implementation.*/
  private List<MyMail> getData(){
    ArrayList<MyMail> list= new ArrayList<MyMail>();
 
    list.add(new MyMail("Test Mail1","TonyQ", 10));
    list.add(new MyMail("Test Mail12","Ryan", 100));
    list.add(new MyMail("Test Mail13","Simon", 15));
    list.add(new MyMail("Test Mail14","Jimmy", 5));    
    return list;
 }
  private List<MyMail> getUpdatedData(){
    ArrayList<MyMail> list= new ArrayList<MyMail>();
 
    list.add(new MyMail("Test Mail1-updated","TonyQ", 10));
    list.add(new MyMail("Test Mail12-updated","Ryan", 100));
    list.add(new MyMail("Test Mail13-updated","Simon", 15));
    list.add(new MyMail("Test Mail14-updated","Jimmy", 5));    
    return list;
 }
 
}

<zk>
 
  <div apply="j3qsost7$v3.TestComposer"> 
    <grid id="inboxGrid" height="350px" 
        rowRenderer="j3qsost7$v3.MyRowRenderer"
        emptyMessage="Nothing in Inbox.">        
        <columns>
            <column  label="Subject" />
            <column width="150px" label="Received" sort="auto(sender)" />
            <column width="80px" label="Size" />
        </columns>
    </grid>
 
    <button label="updated data and refresh" id="refreshBtn" />
  </div>
</zk>