QMatrix vs QWMatrix - porting Qt3 to Qt4

[bitChanger]

void test()
{
  QRect selection(5,5,2,2);
 
#if QT3
  QWMatrix m;
  m.scale(2,5);
  m.invert();
#else
  QMatrix m;
  m.scale(2,5);
  m.inverted();
#endif
 
  QRect r = m.mapRect(selection);
}

void test()
{
  QRect selection(5,5,2,2);

#if QT3
  QWMatrix m;
  m.scale(2,5);
  m.invert();
#else
  QMatrix m;
  m.scale(2,5);
  m.inverted();
#endif

  QRect r = m.mapRect(selection);
}

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Qt 3.3.2
QRect r comes out as:
x1=10
y1=25
x2=12
y2=30

Qt 4.1.2
QRect r comes out as:
x1=10
y1=25
x2=13
y2=34


I'm converting an application from 3.3.2 to 4.1.2 and this seems to be where my problem is coming from.

Does anyone know why this is different and/or how i can fix it.
Thank you.