Because of a system that was agreed to 20 odd years ago.
Our salary is multiplied by a cost of living factor. They say that this year the cost of living in the Netherlands has reduced by 5.2% (politically massaged BS) and so we face the cut, plus these figures are always backdated 12 months, so we have to give 12x5.2% of our salary back too, in a 'lump sum'.