I'm not sure, but I can think of two reasons for this:
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Being a single parent is more expensive than being a couple. Because you can't share costs with another person, a greater proportion of your income goes to required expenses like food, housing, and utilities.
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Along those lines, food is cheaper per person the more people you buy for. Buying in bulk is a huge savings. This is presumably why they give you more money for the first child than for each subsequent child.
Why? Is there a problem with it or is it just a question of too much centralization of users?