On MacOS Qt treats some "standard" MacOS menu items in special way. You are using "Quit" and "About", which are determined by Qt as "standard" for MacOS.
Detailed description and customization details are provided in official Qt docs for QMenuBar http://qt-project.org/doc/qt-4.8/qmenubar.html
Checkout section "QMenuBar on Mac OS X" under the link above.