自定义类中包装的QLabel上出现意外填充

我想在包装QLabel的QWidget周围显示一个边框(这是一个更复杂的小部件的实践)。我正在使用setStyleSheet创建边框。当我手动执行此操作时,它按预期工作。然而,当我将代码移动到它自己的类(从QWidget派生)中时,填充是不同的,我不知道为什么。

import sys
from PyQt5.QtWidgets import QWidget, QLabel, QApplication, QMainWindow, QVBoxLayout 

class WrappedLabel(QWidget):
    def __init__(self, text=''):
        super().__init__()

        self.text = QLabel(text)
        layout = QVBoxLayout()
        layout.addWidget(self.text)
        self.setLayout(layout)
        self.setStyleSheet('padding: 2px; border: 2px solid red;')


class Shell(QMainWindow):

    def __init__(self):  # constructor
        super().__init__()  # call the parent's constructor

        w = QWidget() # Create the main window content widget
        self.setCentralWidget(w)

        # First label
        unwrapped_label = QLabel('This is a normal QLabel with a border and no padding.')
        unwrapped_label.setStyleSheet('border: 2px solid gray; padding: 2px;')

        # Second label
        wrapped_label = QLabel('This QLabel is manually wrapped in a styled QWidget. ' + 
            'There is a slight indent compared to the normal QLabel due to padding.')
        wrapped_layout = QVBoxLayout()
        wrapped_layout.addWidget(wrapped_label)
        manual_wrapper = QWidget()
        manual_wrapper.setObjectName('wrapper')
        manual_wrapper.setLayout(wrapped_layout)
        self.setStyleSheet('QWidget#wrapper { border: 2px solid gray; padding: 2px; }')

        # Third label
        derived_wrapper = WrappedLabel('This class derives from QWidget and wraps a QLabel like above, but is indented even further and the border is in the wrong spot.')

        vbox = QVBoxLayout()
        vbox.addWidget(unwrapped_label)
        vbox.addWidget(manual_wrapper)
        vbox.addWidget(derived_wrapper)
        vbox.addStretch(1) # Squish them together to better see the spacing

        w.setLayout(vbox)

        # Setup the rest of the main window appearance
        self.setGeometry(300,300,640,180)
        self.setWindowTitle('Testing wrapped labels')

        self.show()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    shell = Shell() # create and show the main window
    sys.exit(app.exec_())