The issue is that even after setting the max value on slider, the slider control does not goes to the end value. This is reproducible with the Sencha provided example. In the case of a vertical slider, the slider control itself is not visible.

final Slider s = new Slider();
s.setValue(100);

100 is the max slider value, but the slider does not go end.

Code:
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.IsWidget;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.VerticalPanel;
import com.google.gwt.user.client.ui.Widget;
import com.sencha.gxt.widget.core.client.Slider;
import com.sencha.gxt.widget.core.client.button.TextButton;
import com.sencha.gxt.widget.core.client.event.SelectEvent;
import com.sencha.gxt.widget.core.client.event.SelectEvent.SelectHandler;


public class SliderExample implements IsWidget, EntryPoint {


  @Override
  public void onModuleLoad() {
    RootPanel.get().add(asWidget());
  }


  @Override
  public Widget asWidget() {
    VerticalPanel vp = new VerticalPanel();
    vp.setSpacing(10);


    final Slider s = new Slider();
    s.setValue(100);
    vp.add(s);
    
    TextButton button = new TextButton("Set value to 40");
    button.addSelectHandler(new SelectHandler() {
      
      @Override
      public void onSelect(SelectEvent event) {
        s.setValue(40);
      }
    });
    vp.add(button);
    
    final Slider vs = new Slider(true);
    vs.setIncrement(1);
    vs.setValue(100);
    vp.add(vs);
    
    button = new TextButton("Set value to 40");
    button.addSelectHandler(new SelectHandler() {
      
      @Override
      public void onSelect(SelectEvent event) {
        vs.setValue(40);
      }
    });
    vp.add(button);


    return vp;
  }


}