QFontMetricsF operates differently from platform to platform

If to set font's size in pixels and get QFontMetricsF for this font, then on different platforms, for example, QFontMetricsF::horizontanAdvance( ' ' ) returns different values for the same font for the same QPaintDevice (with the same DPI). Is it known issue?

Maybe you can tell me a workaround? I need the same metrics for the absolutelly same font on all platforms.

For example with

QFont(Droid Serif,-1,83,5,400,0,0,0,0,0,0,0,0,0,0,32768)

const QFontMetricsF fm(font, pd);
const double sp=fm.horizontalAdvance(' ');

returns 22 on Windows and 21.5469 on Linux.