In some situations QFontMetrics gives a smaller value for ::maxWidth() than ::width('X')

I recently was notified about a bug in of my applications where text was being "cut off" After looking into it it seems that QFontMetrics was returning a slightly smaller than expected value for maxWidth(). Since maxWidth() is documented as "Returns the width of the widest character in the font." I would expect that there is no character where width(ch) > maxWidth(), but in at least one setup it is!

For example, on a Debian 6 system the following code:

#include <QApplication>
#include <QFont>
#include <QFontMetrics>

int main(int argc, char *argv[]) {
        QApplication app(argc, argv);
        const QFont f("Monospace", 8);
        const QFontMetrics metrics(f);
        qDebug("MAX_WIDTH=%d : X_WIDTH=%d", metrics.maxWidth(), metrics.width('X'));
}

outputs:

MAX_WIDTH=6 : X_WIDTH=7

So far I have only been able to produce this issue in a "Debian 6 Testing" VM, so this may be an issue with something the debian folks have done. But at least 3 of my users have experienced this. For now, since I only allow fixed width font's in my custom widget, I am working around this issue by simply using ".width('X')"

The version of Qt is 4.6.3 installed from apt-get.