It is mentioned in the docs for "QKeySequence":http://doc.qt.nokia.com/4.7/qkeysequence.html, but if you do not happen to stumble over it, it's unlikely you have read it.
bq. On Mac OS X, references to "Ctrl", Qt::CTRL, Qt::Control and Qt::ControlModifier correspond to the Command keys on the Macintosh keyboard, and references to "Meta", Qt::META, Qt::Meta and Qt::MetaModifier correspond to the Control keys. Developers on Mac OS X can use the same shortcut descriptions across all platforms, and their applications will automatically work as expected on Mac OS X.
BTW: Qt just switches to the Mac's standard behavior here - so, it is not really a surprise. A Mac user without knowledge of Qt's docs would have done it the correct way intuitively :-)