The Kindergeld Mystery

In Germany there is a monthly transfer payment that parents get from the state called Kindergeld. There are two justifications I can think of for this, namely fairness (or whichever similar term you prefer) and incentivization. Kindergeld is € 154 per month for each of the first three children and €179 for each subsequent child. Why the progression?

It doesn't make any sense in terms of incentivization because people who already have three children have already demonstrated their preference for having children at € 154/month (- and see next paragraph).

It doesn't make any sense in terms of fairness. If anything, subsequent children are cheaper because they can use their elder siblings' old toys, clothes, etc. and a flat's or house's square metres have a decreasing marginal price, for whatever reason.

Someone who knows a lot about these things had no clue why this state of affairs persists. Nor do I.