なんでupdateじゃなくてrepaintなの？

あと http://qt-project.org/doc/qt-4.8/qwidget.html#paintEvent の
> Generally, you should refrain from calling update() or repaint() inside a paintEvent().
> For example, calling update() or repaint() on children inside a paintevent()
> results in undefined behavior; the child may or may not get a paint event.
は関係あるのだろうか