In case anybody is interested, it's because you qualify for the play-off if you win one of mini-leagues that take place. So you can finish 17th, but if you have a good run over a 6 game period, you can still qualify for the play-offs.
"The competition is divided into six periods (Dutch: periode) of six matches each. The winner of each period (Dutch: periodekampioen) qualifies for the playoffs at the end of the season. If the winner of a period has already won a prior period in the season, the second placed team in the period is awarded the playoff slot. If the second placed team has also won a prior period, no winner is called, and the playoff slot is decided by league standing at the end of the season."
And I thought the split was too complicated