Can not move horizontalScrollBar to the rightmost side

[feiyuhuahuo]

class PP(QMainWindow):
    def __init__(self):
        super().__init__()
        loader = QUiLoader()
        self.main_ui = loader.load('test.ui')
        self.setCentralWidget(self.main_ui)
        self.main_ui.pushButton.clicked.connect(self.add_l)
        self.resize(500, 150)
        self.show()

    def add_l(self):
        print(self.main_ui.scrollArea.horizontalScrollBar().maximum())
        self.main_ui.horizontalLayout_2.addWidget(QLabel('  ***aaaaa**  '))
        print(self.main_ui.scrollArea.horizontalScrollBar().maximum())
        print('--------------')
        self.main_ui.scrollArea.horizontalScrollBar().setValue(9999)


if __name__ == '__main__':
    app = QApplication()
    ui = PP()
    app.exec()

I'm using the above code to do a test.
The 'test.ui' is a very simple UI created by Qt Designer.

Every time I push the button, a new QLabel is add to the layout. And at the same time, I try to move the horizontalScrollBar to the rightmost side. But it failed. There's always some space left at right.

The max value of the horizontalScrollBar did not change after I add a QLabel.