a bug for setContentsMargins()

I make a qt gui application using the wizard.

#include <QLabel>
#include <QStatusBar>

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

setContentsMargins(20, 0, 60, 0);

QLabel *lable1 = new QLabel(tr("abc"));
statusBar()->addWidget(lable1);
}

And you will see the layout of the statusbar is not correct.