The width of the rect returned by QFontMetrics::boundingRect(QString) is to small on Mac OS X

The boundingRect() overloads return rects with different widths. Reproduced on OS X Leopard with Qt 4.5-main

Code:
QString string = QString::fromUtf8("ششسييبزشسيث");
QFontMetrics fm((QFont()));
qDebug() << fm.boundingRect(string);
qDebug() << fm.boundingRect(0,0,0,0,0, string);
qDebug() << fm.boundingRect(QRect(),0, string);

Output:
QRect(0,-13 39x16)
QRect(0,0 74x17)
QRect(0,0 74x17)

QFontMetrics:width() also returns 39.