I think I start to understand. It seems sub-pixel positionning was implemented in Qt 4.8 and is active when hinting is turned off (or 'slight'). On my Ubuntu box, this is probably the case and I can see the problem. On my Debian sid box, I have even multiple of 10 as well, so I suppose it is due to a different setting of hinting.

I am still unsure about the relationship between "hinting" and "subpixel positionning" though.

This blog post talks about that but I'm no typography expert: http://labs.qt.nokia.com/2011/03/14/...e-say-no-more/

In any case, I found QFont::setStyleStrategy( QFont::ForceIntegerMetrics ) that seems to do exactly what I want: turn off sub-pixel positionning regardless of hinting. Works perfectly and I am back with integer widths.