Question about child support in Arizona

Does anyone know why Arizona cuts child support off after the 6th child?

I know a couple who are splitting up and the mother will have primary custody. They have 7 kids. The father (and yes, he is the father of all seven, he does not contest that) will only have to pay child support for the first six. The state calculates the amount of child support due the primary custodial parent based on whatever their calculations are up to six kids. Once the oldest turns 18, the youngest can be added on so the mother is still receiving support for six kids, but in the mean time, she is raising all seven and he is supporting only 6 of them.