the divideScale() of QwtLogScaleEngine

hi,everyone
when I use the class QwtLogScaleEngine,I have a question.
Following is the code:

Code:

---

QwtScaleDiv div(1,1000);
    QwtLogScaleEngine *logSE = new QwtLogScaleEngine();
    div = logSE->divideScale(1,1000,5,5);
    qDebug() << "log major ticks" << div.ticks(QwtScaleDiv::MajorTick);
    qDebug() << "log medium ticks" << div.ticks(QwtScaleDiv::MediumTick);
    qDebug() << "log minor ticks" << div.ticks(QwtScaleDiv::MinorTick);

---

the debug message is following:
log1 major ticks (1, 10, 100, 1000)
log1 medium ticks ()
log1 minor ticks (4, 6, 8, 40, 60, 80, 400, 600, 800)

My question is that why the minor ticks don't contain the 2,20,200 ?I can't understad it.I have search the source code of class QwtLogScaleEngine,but I can't understand it clearly.
Thanks in advance!!