It seems that there is a mistake on the QLineF class reference page (http://qt-project.org/doc/qt-4.8/qlinef.html):
“The intersect() function determines the IntersectType for this line and a given line, while the angle() function returns the angle between the lines”
It should be angleTo() instead of angle(), because in accordance with documentation:
angle() – Returns the angle of the line in degrees
angleTo(const QLineF & line) – Returns the angle (in positive degrees) from this line to the given line.